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3o, to- 2 



[The sole rights of translation in English remain with Scott, Green-wood ? Sen.\ 




ORIGINALLY issued as a volume of the series on 
pigments and colouring matters by the present author's 
father, the necessity for a new edition afforded a welcome 
opportunity of revising "Earth Cofours." Although, 
in the nature of things, little progress has been made 
in this subject itself, there was a good deal to add in 
connection with the mechanical appliances for treating 
the colour earths and manufacturing them into pigments. 
In other respects, too, the work has been carefully 
gone through and brought up to date, with new and 
additional illustrations. 

The author desires to express his thanks to the 
various firms who have afforded him assistance in his 
task by furnishing illustrations and descriptions of new 
machinery, together with other information. It is hoped 
that this third edition will meet the approval of those 
interested in the subject ; and the author will be glad to 
receive supplementary information to render the work 
more complete in the event of a future edition being 
found advisable. 








(A) White Raw Materials and Pigmentary Earths . 1 1 
Limestone (Calcite, Limestone, Chalk) . . 1 1 
Gypsum (Alabaster) . . . . .18 
Barytes, or Heavy Spar . . . 19 
Talc, Soapstone, Steatite . 20 
Clay . . . . . .21 

(B) Yellow Earths . . . . . .23 

Brown Ironstone . . . . ,23 

Ochre ....... 25 

Yellow Earth 26 

Terra di Siena . . ... . .27 

(C) The Red Earths 27 

Red Ironstone . . . . . .28 

Bole 31 

Alum Sludge . . .. . . .32 

Mine Sludge ...... 32 

(D) Blue Earths 33 

Azurite, or Ultramarine . . . , -33 
Vivianite ....... 33 





(E) Green Earth Pigments .... 34 

Green Earth ...... 34 

Malachite . . . . . -35 

(F) Brown Earth Pigments .... 36 

Umber ....... 36 

Asphaltum . . . . . -37 

(G) Black Earth 38 

Black Schist ... 38 

Graphite ....... 38 


Crushing Machinery . . . . . . 43 

Crushing and Sifting . . . . . -77 

Calcining . . . . . . .81 

Mixing and Improving . . . . .81 

Moulding ....... 85 



Caustic Lime ....... 87 

Pearl White ,94 

Vienna White ....... 95 

Chalk 98 

Precipitated Chalk . . . . . . 107 

Calcareous Marl . . . . . .no 

Gypsum . . . . . . . .in 

Kaolin, Pipeclay . . . . . .112 

Bary tes, or Heavy Spar . . . . .119 

Carbonate of Magnesia . . . . .123 

Talc . . . . . . , .124 

Steatite or Soapstone . . . . .125 





The Ochres 128 

Calcining (Burning) Ochre .... 132 

Ochres from Various Deposits . . . .136 

Artificial Ochres . . . . . .138 

Ochres as By-products . . . . .146 


RED EARTH COLOURS . . . . . -151 

Bole 152 

Native Ferric Oxide as a Pigment . . 154 

Iron Glance ....... 154 

Hematite . . . . ... 155 

Raddle . . . . . . . .155 

Burnt Ferric Oxide and Ochres . . . .158 

(a) Burning in the Muffle . . . .158 

(b) Caput Mortuum, Colcothar . . .160 

(c) Calcining Ferric Oxide . . . .161 
Ferric Oxide Pigments from Alum Sludge . .164 



Terra di Siena 168 

True Umber . . . . . . .170 

Cologne Earth (Cologne Umber) . . .173 

Asphaltum Brown (Bitumen) . . . .174 



Green Earth, or Celadon Green . . .176 

Artificial Green Earth (Green Ochre) . . .180 

Malachite Green . 181 




BLUE EARTH COLOURS . . . . . .183 

Malachite Blue (Lazulite) 183 

Vivianite or Blue Ochre . . . . .184 


BLACK EARTH COLOURS. . . . . .185 

Graphite . . . . . . .185 

Black Chalk 194 



COLOURS . . . . . . -197 

White Earth Colours 198 

Yellow Earth Colours . . . . .200 

Red Earth Colours . . . . . .200 

Brown Earth Colours ..... 200 

Green Earth Colours ..... 201 

Blue Earth Colours ...... 202 

Grey Earth Colours . . . . . . 202 

Black Earth Colours . . . . .202 

INDEX . . . . . . . , 203 




BOTH from the chemical and practical standpoint 
it is necessary to divide pigments into clearly denned 
groups, the following classification being adopted on 
the basis and natural history of the substances con- 
cerned : 

(i) Pigments occurring native in a finished con- 
dition, and only requiring mechanical preparation to 
fit them for use as painters' colours. (2) Pigments 
which are not ready formed in Nature, but contain 
some metallic compound as pigmentary material, 
which requires certain chemical treatment for its full 
development. (3) Pigments which, in contrast to 
these two groups, contain only organic, and no in- 
organic, constituents. This last class comprises all 
the natural vegetable pigments, together with the 
large group of colours obtained artificially from tar 
products, fresh groups of which are being continually 
introduced. Nowadays, there is no longer any strict 
line of demarcation between the natural and artificial 
organic colouring matters, it being possible to produce 
even those of the vegetable series, such as madder 
and indigo, by artificial means. 


Whilst this group of colours exhibits the greatest 
variety, and is constantly being enriched and increased 
by the progress of colour chemistry, the case is different 
with the first group, the natural earth pigments. Here 
we have chiefly to do with the preparation of materials 
occurring in Nature, or with bringing about certain 
chemical results, so that, consequently, the range of 
variety is far more restricted, and there is little or no 
possibility of increasing the number of these colours 
by the manufacture of really new products. The 
earth colours nevertheless have a high technical and 
economic importance, on account of their extremely 
valuable properties, coupled, for the most part, with 
low cost. 

If the term " earth colours " were, strictly adhered 
to, the present work would have to be confined to a 
description of the physical and chemical properties 
of the various pigments, and of the various means by 
which they can be brought into suitable condition 
for use in paints. 

However, of late, the term has found wider applica- 
tion than formerly, since it has been found practicable 
to modify (shade) certain of the earth colours by simple 
operations, and thus considerably increase the range 
of tones of the substances known as earth colours. 
The progress of chemical industry has also largely 
increased the number of the so-called earth colours, 
certain methods of chemical treatment having enabled 
substances that are of little use for other purposes, 
to be employed, in large quantities, as pigments. The 
application of these usually cheap by-products is 
still further facilitated by the fact that they can be 
transformed, by a simple chemical treatment, into 
pigments which are distinguished by their beauty of 


colour and at the same time possess the great advantages 
of durability and cheapness. 

As an example of this, mention may be made of 
iron oxide, which occurs in Nature in the form of various 
minerals which can be made into pigments by mechanical 
treatment. In many cases, this treatment has already 
been carried out by Nature, and deposits of iron oxide 
are found in which the material has only to be incor- 
porated with a vehicle to make it fit for immediate 
use as a painters' colour. 

Moreover, the same oxide is obtained, in large quan- 
tities, as a by-product of the treatment of other minerals. 
From the point of view of chemical composition, this 
by-product is of very low value, by reason of the large 
supplies of native oxide available. By means of a 
very simple chemical treatment, however, this by- 
product oxide can be considerably improved in com- 
mercial value, being, in many cases, convertible, by 
merely heating it to certain temperatures, into a variety 
of colours which sell at remunerative prices. 

Consequently, in view of the present condition of 
the chemical industry, the term " earth colours " 
can be enlarged to include a number of waste products 
which fetch good prices as colours, though otherwise 
practically valueless in themselves. 

The number of earth pigments is very large, and 
comprises representatives of all the principal colours. 
For painting purposes, few pigments beyond the earth 
colours were known to the ancients ; and most of the 
colours in the paintings which have come down to us 
from antiquity are pure earth pigments, thus affording 
proof of their great durability, having retained their 
freshness unimpaired for hundreds and some for 
thousands of years. 


The earth colours might be divided into such as 
occur ready-formed in " Nature, and require only 
mechanical preparation, and which either require 
special treatment (e. g. calcining), or are artificial 
products (like the iron oxide mentioned above). Since, 
however, such a classification would not advantage 
our knowledge of the nature of this class of colours, 
it appears useless and superfluous, and we will therefore 
simply confine ourselves to arranging the earth pig- 
ments according to their colour white, yellow, red, etc. 

Adopting this classification, the following minerals 
and chemical products may be considered as earth 
colours : 

White. These include the varieties of calcium 
carbonate, such as chalk, marble, precipitated chalk, 
calcium phosphate, calcium sulphate (in the form of 
gypsum, alabaster, muriacite and the precipitated 
gypsum produced as a by-product in many chemical 
works), heavy spar, the different varieties of clay, and 

Yellow. This group comprises ferric hydroxide 
(hydrated oxide of iron) in the form of the various 
minerals known as ochre ; all the preparations chiefly 
composed of this hydroxide, and all those prepared 
by artificial means. A very important member of this 
group is orpiment ; the other arsenical compounds 
frequently met with native, being however, on account 
of their poisonous properties, no longer used as pigments. 

Red. Chief among the red earth colours are those 
consisting of ferric oxide (iron oxide), under various 
names. The only other member of the group is the 
far rarer vermilion. 

Blue. The blue earth colours are few in number 
and of no particular beauty ; but they are of importance 


on account of their cheapness and because all the 
artificial blue pigments are rather expensive. Two 
products in particular merit attention in this con- 
nection, namely, ultramarine, and the mineral known 
as blue ochre or blue ironstone. The latter, as a matter 
of fact, cannot be used for anything else than a painters' 
colour, and can be obtained at a low price; whereas 
ultramarine also forms a valuable raw material for 
the recovery of copper, and is therefore dearer. 

Green. This group, again, contains only two mem- 
bers, viz. malachite green (chrysocolla), and the green 
earths (seladonite), known as Verona, etc. , green. These 
occur fairly often in Nature, and the green earths in 
particular find a wide industrial application by reason 
of their low price. Malachite green is very similar, 
in chemical constitution, to ultramarine ; and both 
form sources of copper and are consequently expensive. 

It should be mentioned that both ultramarine and 
malachite green can only be profitably made into 
pigments where the minerals can be obtained cheaply, 
since both of them can be manufactured where arti- 
ficial pigments are produced, and are put on the market 
under the same names as the native articles. The 
very low price of the green earths makes them highly 
popular as colouring matters in certain branches of 
industry, and they are very largely used by wall-paper 

Brown. This is a large group, and the pigments 
composing it are specially distinguished for their beauty 
and depth of colour, on which account they are used 
in the finest paintings. Here, again, it is ferric oxide, 
in combination with water and therefore ferric 
hydroxide that furnishes a large number of the mem- 
bers of the group. Like the renowned Siena earth, 


the artists' colours known as Vandyck brown, bole, 
Lemnos earth, umber, etc., mainly consist of more 
or less pure ferric hydroxide. These minerals are, 
moreover, specially important to the colour manu- 
facturer, inasmuch as most of them enable a large 
number of different shades to be obtained by a simple 
method of treatment consisting merely of the applica- 
tion of heat in a suitable manner; and these colours 
are among the most excellent we possess, by reason 
of their beauty and permanence. Amongst this series 
must also be classed native manganese brown, which 
chiefly consists of a mixture of manganese oxide and 
the hydrated peroxide of the same metal. 

Black. There is really only one member of this 
class, which, however, is frequently used, viz. that 
form of carbon occurring as hexagonal crystals and 
known as graphite. Another natural black natural 
product, occasionally used as a painters' colour is 
the so-called black chalk. However, since black 
pigments can be produced very cheaply by artificial 
means, the natural colours find only a limited applica- 
tion ; and only in one instance is graphite used alone, 
viz. for making blacklead pencils. 

As already mentioned, certain chemical industries 
furnish by-products which are of very little value 
in themselves, and many of them, indeed, may be 
classed as worthless, since chemical manufacturers 
naturally endeavour to get everything possible out 
of their materials in the course of manufacture. 

Some of these by-products, however, can advantage- 
ously be used as pigments, a good example of this 
being afforded by the iron oxide formed as a by-product 
in the manufacture of fuming sulphuric acid (Nord- 
hausen oil of vitriol), by the old process, from green 


vitriol (ferrous sulphate). In itself, this oxide is 
practically valueless, but, by very simple treatment, 
it can be converted into very valuable pigments which 
have a market value far in excess of the original material. 
Although it has hitherto been the custom to confine 
the term earth colours to such as occur ready-formed 
in Nature and only require simple mechanical treatment 
to make them ready for immediate use as pigments, 
the author is nevertheless of opinion that a book dealing 
exhaustively with earth colours should also make some 
mention of all the mineral colouring matters which 
can be easily made into pigments by simple processes, 
such as calcination or bringing into association with 
other substances. In accordance with this view, the 
present work will describe all the pigments that are 
obtainable in this manner. Most of the earth colours 
consist of decomposition products of certain minerals ; 
and this applies particularly to such of them as contain 
iron oxide. According as the decomposition of the 
original mineral has been more or less extensive, the 
natural product exhibits different properties; and 
the manufacturer must consequently endeavour to 
treat them in such a manner as to ensure that the pig- 
ment obtained will be as uniform as possible in shade 
and permanence. In order to accomplish this it is 
essential to have an accurate knowledge of the origin 
of the raw material under treatment, and of its chemical 
and physical properties. In view of this, the author 
considers it necessary to deal more fully with the pig- 
mentary earths forming the raw materials of the earth 
colours, before passing on to the preparation of the 
colours themselves. 



THE minerals constituting the raw materials for the 
preparation of the earth colours occur under very 
divergent conditions in Nature. Some of them, such 
as chalk, form immense deposits, even whole mountains, 
whilst in other cases, e. g. the blue ferruginous earths, 
the occurrence is connected with certain local con- 
ditions, and many are found only in isolated deposits, 
as pockets or beds. This last is the case, for instance, 
with the handsome brown iron pigments ; and indeed 
the names by which they are known indicate that they 
are only found in well-defined localities, or that they 
are met with of special quality there. The brown 
earth colour known to all painters as Terra di Siena, 
is found at many other places as well as near Siena, 
but the product from that city acquired aforetime 
a special reputation for beauty, and therefore all 
similar earths, provided they are equal to that from 
Siena, also bear the same name in commerce. 

A number of raw materials for the preparation of 
earth colours are found, it is true, in many deposits, 
but their utilisation depends, in turn, on local con- 
ditions. For example, many copper mines contain, 
in addition to the other cupriferous minerals, those 
used, in the powdered state, as ultramarine or ultra- 
marine green, and not infrequently lumps of mineral 


are found containing both blue and green together. 
However, it is only when these minerals occur in 
sufficient quantity to make the necessary sorting 
profitable that their manufacture into pigments can 
be regarded as practicable. 

Before commencing to work a deposit it is essential 
to make sure whether the raw material, or pigmentary 
earth, is actually suitable for the manufacture of earth 
colour. Even the general character of the material 
is important, those of soft, earthy consistency being 
much easier to treat, and the cost of preparation 
smaller, than if the raw material be hard, tough and 

The extent and thickness of the deposit, and the 
ease with which it can be worked, also play an im- 
portant, and even decisive part, since, other conditions 
being equal, it will not pay to erect a colour works 
unless the raw material is available in sufficient quantity 
and is cheap. Generally, the deposit is not homo- 
geneous throughout, the mineral being purer in some 
places and more contaminated with gangue in others. 
The percentage of moisture also varies, and in short, 
a number of circumstances must be taken into con- 
sideration in forming a conclusion as to whether a 
deposit is workable or not. 

In order to arrive at a reliable opinion on all these 
conditions, sampling is indispensable. If the samples 
are of uniform character, they can be mixed together 
to make an average sample. But if they differ con- 
siderably in appearance, general character, proportion 
of gangue, etc., it is preferable to examine them 
separately, more especially when the area which each 
represents is large. 
The examination should extend, on the one hand, 


to the natural percentage of moisture, and, on the 
other, to the purity of the material. The water con- 
tent is determined by thoroughly drying a weighed 
sample, bearing, however, in mind the fact that 
pigmentary earths of a clayey nature vary in water 
content according to the time of year, besides changing 
in accordance with the weather when the won material 
is stored in the open. 

The purity can only be ascertained by an examination 
in which a sample of the soft, clayey material is crushed 
and passed through a narrow-mesh gauze sieve, the 
amount of the coarse particles sand, small stones, 
etc. remaining on the sieve being determined. A 
more accurate method, of course, is to separate the 
true pigmentary earth from the gangue by levigation. 
For this purpose, a weighed quantity of the crushed, 
air-dry sample is placed in a relatively narrow glass 
vessel and thoroughly mixed with water, the turbid 
supernatant liquid being poured off after a short 
interval. The residue is repeatedly treated in the 
same way, until no more fine particles remain in 
suspension, the residue then consisting of impurities, 
or gangue. Of course, the washings can be collected, 
the suspended matter allowed to settle, and finally 
weighed in an air-dry condition. By this means an 
approximate idea of the yield of earth colour can be 
obtained at the same time. 

Raw materials which are not amorphous, soft and 
clayey must first be crushed, an operation facilitated 
by heating to redness and quenching in cold water. 
Oftentimes the heating causes a change of colour 
and improves the covering power a point to which 
reference will be made later on. 

In the following description of the various raw 


materials, the chemical composition of the pure 
minerals will be given, together with an enumeration 
of the most common impurities. 


Limestone (Calcite, Limestone, Chalk) 

The number of materials furnishing white earth 
colours is comparatively large, and these colours are 
particularly important, because, not only are they 
extensively used by themselves, but they also serve 
as adjuncts to other colours and for the production 
of special shades. The chief raw material for the 
preparation of white earth colours is the mineral 
calcite in its numerous modifications. 

Calcite, or calc spar, occurs very frequently in 
Nature, and is one of the most highly diversified 
minerals known. In its purest state it appears as 
" double spar " (calcite), in the form of water- white 
crystals, which are very remarkable, for certain optical 
properties. White marble is also a very pure variety 
of calcite, in which the individual crystals are very 
small. The various coloured marbles owe their appear- 
ance to certain admixtures of extraneous substances, 
chiefly metallic oxides. 

No sharp line of demarcation separates marble from 
ordinary limestone, the difference between them 
really consisting only in the degree of fineness of 
grain ; and all limestones which grind and polish well 
may be classed as marble. As is the case with marble, 
there are also limestones of various colours, grey being, 
however, the most common. This grey limestone 
forms huge mountain masses which, in Europe, follow 


for example, the Alpine chain on its northern and 
southern edges. 

A few other examples of calcite may be mentioned 
which occur in certain localities and, in part, are still 
in course of formation. To these belong the stalactites 
and stalagmites, which sometimes consist of extremely 
pure calcite. They are formed by the action of water, 
containing carbonic acid in solution, which trickles 
through cracks and cavities in limestone rock and 
dissolves out calcium carbonate from the adjacent 
stone. On prolonged exposure to the air such water 
gives off its free carbonic acid again ; and as the calcium 
carbonate is insoluble in pure water, it separates out 
in crystalline form. The masses formed in this way 
usually resemble icicles in shape, and the finest 
examples are to be found in the well-known stalactite 
grottoes at Krain, whilst the grotto at Adelsberg is 
renowned for its beautiful stalactites. Occasionally, 
stalactites have an opaque yellow or brownish tinge, 
which they owe to the presence of iron oxide. 

A formation similar in its origin to stalactites is the 
so-called calc sinter and calcareous tuff. The former 
often occurs in cavities as irregular masses which, in 
some places, enclose large quantities of fossil animal 
bones, in which case they form " bone breccia " 
(crag breccia). Calcareous tuff is deposited from 
numerous springs, occasionally in very large quantities, 
enveloping plants and sometimes forming thick deposits 
in which the structure of the plants can be clearly 

In some places a more or less pure white, extremely 
friable variety of calcite is met with under the name 
"mountain milk" or "mountain chalk" (earthy 
calcite), which seems to be a decomposition product, 


and consists of a mixture of arragonite and chalk. 
Arragonite which will be referred to later is com- 
pletely identical, chemically, with calcite both being 
composed of calcium carbonate the sole difference 
being their crystalline form. 

The most important for the colour-maker, however, 
is the variety known as chalk. This is really a fossil 
product, i. e. it consists of the microscopic shells of 
marine animals united into solid masses. Despite 
the smallness of these animals, their epoch lasted long 
enough for their shells to form entire mountains which 
are encountered all over the world. A large part of 
the coast of England, the island of Riigen, and many 
other localities, consist entirely of chalk. 

In many cases, chalk is found interspersed with 
nodular masses of flint, and in some places it also 
contains great quantities of the remains of other 
marine animals, such as sea urchins, the spines of which 
occur in such numbers in certain kinds of chalk as to 
unfit them entirely for use as a pigment. 

The foregoing varieties of calc spar are the most 
important, and also occur in large quantities ; but, to 
complete the tale, it is necessary to mention also a few 
others which, however, are only found in small amounts. 
To these belong, for example, anthracolite, a limestone 
stained quite black by coal ; the oolithic limestones or 
roe stones, which are composed of granules resembling 
fish roe ; muschelkalk, which is also of fossil character 
and is almost entirely composed of mussel shells 
cemented together with lime ; the marls, which consist 
of calc spar mixed with varying quantities of clay and 
consequently often bear a great resemblance to loam 
in their properties. A few of these varieties find 
extensive employment for certain purposes, some 


marls for instance being used for making hydraulic 
lime, whilst all modifications of calc spar that are 
sufficiently pure can be burned for quick lime. 

It has already been stated that the mineral arragonite 
is. identical, chemically, with calc spar, since both 
consist of calcium carbonate, but differ in their crystal- 
line habit. Thus, whereas the crystals of calc spar 
belong to the rhombohedral or hexagonal system, those 
of arragonite are always rhombic. This occurrence of 
one and the same substance in two different crystalline 
forms is known as dimorphism, and calcium carbonate 
is therefore dimorphous. Whether calcium carbonate 
assumes the form of calcite or arragonite depends 
entirely on physical causes. When the deposition of 
the carbonate takes place from a cold solution the 
shape of the crystals is always one belonging to the 
hexagonal or rhombohedral system; but when it 
is from hot solution, rhombic crystals are invariably 
formed, calc spar resulting in the former case and 
arragonite in the latter. 

These different methods of formation which can be 
carried out in the laboratory by producing the re- 
quisite conditions, occur on the large scale in many 
parts of the world. Wherever a hot spring comes to 
the surface, containing considerable amounts of lime 
in solution, this separates out in the form of arragonite, 
which received its name from the circumstance that 
specially handsome crystals of this mineral are found 
in Arragon. 

One of the best-known places where the formation 
of arragonite can be observed at the present time is 
Carlsbad in Bohemia. The hot springs there deposit 
a very large amount of lime, which is stained more 
or less yellow or red by the presence of varying quan- 


titles of iron oxide, and, under the name of" sprudel- 
stein " is used for producing various works of art. 
When the hot springs bring up particles of sand, the 
lime substance incrusts these sand grains, forming 
globular masses resembling peas, and consequently 
named pisolite. 

In chemical composition, calcite and arragonite 
consist of a combination of calcium oxide (lime) and 
carbonic acid, the formula being expressed by CaCO 3 . 
Calcium carbonate is insoluble in pure water, but 
dissolves somewhat freely in water charged with free 
carbonic acid. It is assumed that a compound is 
formed, which is known as calcium bi- (or acid) car- 
bonate, is very unstable and can only exist in a state 
of solution. When a solution of calcium bicarbonate 
which can be prepared by passing carbonic acid gas 
through water containing finely divided calcium 
carbonate in suspension is exposed for some time to 
the air, it soon becomes cloudy, and a deposit of calcium 
carbonate settles down at the bottom of the vessel, 
because, in the air the dissolved calcium bicarbonate 
is decomposed into free carbonic acid gas and calcium 
carbonate, which latter, as has been mentioned, is 
quite insoluble in water. It has already been stated 
that this phenomenon goes on in Nature in the forma- 
tion of stalactites, lime sinter and calcareous tuff. 

Calcium carbonate is readily soluble in acids, the 
contained carbonic acid being liberated (as carbon 
dioxide) with effervescence. WTien such acids are 
employed for solution as form readily soluble salts 
with lime, such as hydrochloric, nitric, acetic, etc. 
acids, a perfectly clear solution is obtained; but if 
sulphuric acid is used, a white pulpy mass is formed, 
consisting of calcium sulphate, or gypsum, which, 


owing to its low solubility, separates out as small 
crystals. Any sandy residue left when calcium 
carbonate is dissolved, mostly consists of quartz sand. 
In dissolving dark -coloured limestones, grey, or even 
black, flakes are left, which consist of organic material 
very high in carbon. On limestone being subjected 
to fairly strong calcination, all the carbonic acid is 
expelled, leaving behind the so-called quick or burnt 
lime, which is, chemically, calcium oxide : 

CaCO 3 = CaO + CO 2 

Calcium Quick Carbon 

carbonate lime dioxide 

If burnt lime be left exposed to the air for some 
time, it again gradually absorbs carbon dioxide and 
is reconverted into calcium carbonate. When burnt 
lime is sprinkled with water it takes up the latter 
avidly, becoming very hot and finally crumbling down 
to a very friable white powder, consisting of slaked 
or hydrated lime (calcium hydroxide, Ca(OH) 2 ). The 
considerable rise of temperature in quenching the lime 
is due to the chemical combination of the calcium oxide 
and water. 

Both quick and slaked lime dissolve to a certain 
extent in water, and impart strongly alkaline properties 
thereto, lime being one of the strongest of bases. On 
exposure to the air, the solution of quick lime in 
water (lime-water) quickly forms an opalescent super- 
ficial film of calcium carbonate, and in a short time no 
more lime is present in solution, the whole having been 
transformed into calcium carbonate, which settles 
down to the bottom of the vessel as a very fine powder. 

Limestone that consists entirely of calcium oxide 
and carbon dioxide is of rare occurrence in Nature, 


foreign substances being nearly always present. Since 
the nature of these admixtures is of the greatest 
importance to the colour-maker, owing to the consider- 
able influence they exert on the suitability of the 
minerals for his purposes, it is necessary that these 
extraneous substances occurring in limestone should 
be more closely described. 

Nearly all varieties of limestone contain certain 
proportions of ferrous and ferric oxides. The presence 
of ferrous oxide, when the relative amount is but 
small, cannot be detected by mere inspection ; and 
even many limestones containing really appreciable 
quantities of ferrous oxide are pure white in colour 
so long as they are in large lumps. If, however, such 
a limestone be reduced to powder and exposed to the 
air for a short time, it gradually assumes a yellow 
tinge, the depth of which increases with the length 
of exposure. 

The cause of this change is due to the fact that 
ferrous oxide has a great affinity for oxygen, by absorb- 
ing which it changes into ferric oxide. (Ferrous oxide 
consists of FeO, ferric oxide of Fe 2 O 3 .) Ferrous oxide 
and its compounds are of a pale green colour which 
is not very noticeable, whereas ferric oxide has a very 
powerful yellow colour, and consequently the lime- 
stone, when its superficial area has been greatly 
increased by reduction to powder, assumes the yellow 
tinge due to ferric oxide. A limestone exhibiting 
this property can evidently not be used for making 
white earth colours, but is, at best, only suitable for 
mixing with other colours. 

Occasionally, limestone contains varying quantities 
of magnesia, and when this oxide is present in large 
amount, changes into another mineral known as 



dolomite. In many places this dolomite forms large 
masses of rock, which, however, is not employed for 
making colours, owing to the yellow shade imparted 
by the fairly large amount of ferric oxide present. 

Gypsum (Alabaster) 

This mineral occurs native in many places, and is 
frequently worked for a number of purposes. Gypsum 
occurs in Nature in a great variety of forms. The 
purest kind is met with either as water-clear crystals, 
which cleave readily in two directions, or as trans- 
parent tabular masses (selenite) which also cleave easily. 
Micro-crystalline fine-grained gypsum is milk-white in 
colour, highly translucent and is largely used, under 
the name of alabaster, in sculpture. Owing to its 
low hardness, alabaster can be readily cut with a knife, 
and on this account is frequently shaped by planing or 

Gypsum is generally met with in dense masses, 
which may be of any colour, grey, blue and reddish 
shades being the most common, whilst pure white is 
rarer. The dark-coloured varieties can only be used 
for manurial purposes ; but the white finds a two-fold 
application as a pigment, and, in the calcined state, 
for making plaster casts. 

In point of chemical composition, gypsum consists of 
sulphate of lime, or calcium sulphate (CaSO 4 -f 2H 2 O). 
It is soluble in water, but only in such small quantity 
that over 400 parts of the latter are needed to dissolve 
one part of gypsum. On being heated to between 
120 and 130 C., gypsum parts with its two molecules 
of combined water and becomes anhydrous calcium 
sulphate or burnt gypsum. When this 'latter is stirred 
with water to a pulp, it takes up the water again, with 


considerable evolution of heat, swelling up considerably 
and setting quickly to a solid mass. 

The number of substances exhibiting this property 
being small, burnt gypsum is very frequently used for 
making casts of statuary, and for stucco work in 
building. Finely ground white gypsum can also be 
used as a pigment, but is inferior to calcium carbonate 
in covering power, and is therefore seldom employed 
for this purpose, though frequently added to other 
colours. The mineral known as muriacite or anhydrite 
consists of anhydrous calcium sulphate ; and is there- 
fore similar in composition to burnt gypsum; but it 
lacks the property of combining with water when 
brought into contact therewith. 

Barytes, or Heavy Spar 

The mineral known as heavy spar occurs in very 
large quantities and in numerous localities. It forms 
rhombic crystals, which are very often extremely 
well developed and form flat plates of considerable 
size. A remarkable peculiarity of this mineral is its 
high specific gravity, which is due to the barium 
content. It is found native in all colours, white being 
the most common. 

Chemically, heavy spar is barium sulphate, BaS0 4 . 
It can be used as a pigment per se, but only when 
prepared artificially, the trade name for the product 
being permanent white, or blanc fixe. Powdered 
native heavy spar, even when ground ever so fine, has 
not enough covering power, this property being 
peculiar to the artificial product. 

When it is desired to mix other pigments with a 
white substance, to lighten the shade, permanent 
white can be specially recommended, since it is quite 


insensitive to atmospheric influences and has no 
chemical action on the colour, so that it can be used 
with even the most delicate colours without risk. In 
this way, not only can the colours be considerably 
cheapened, but over -dark colours can be shaded to 
the desired extent. Another advantage of such 
mixtures is that a smaller quantity of oil or varnish 
is required, barytes only needing about 8% of its own 
weight of vehicle to form a workable mixture, whilst 
other pigments take five times as much, or even more. 
In many cases the low covering power of barytes 
enables large quantities to be added, and this reacts 
favourably on the consumption of varnish. 

Another barium mineral is witherite, or barium 
carbonate. This is not used direct as a pigment, 
but in contrast to heavy spar is readily soluble in 
hydrochloric acid, and therefore serves as raw material 
for the preparation of artificial barytes and other 
barium compounds, the first -named being obtained 
by treating a solution of barium chloride with sulphuric 
acid, insoluble barium sulphate being precipitated. 

Talc, Soap stone, Steatite 

Talc occurs in Nature either as a pure white' mass, 
of greasy lustre, or occasionally as yellow, green or 
grey masses, all distinguished by a peculiar greasy 
appearance and a soapy feel. This appearance is 
common to all the minerals of the steatite group, and 
is the cause of their generic name, soapstone. Although 
the steatites have a very low degree of hardness 
most of them can be scratched by the finger-nail- 
some difficulty is encountered in reducing them to 
fine powder. Calcination usually increases the hard- 
ness considerably, so that, in some cases, the calcined 


mineral gives off sparks when struck with a steel 

Soapstone is composed of magnesium silicates, 
containing varying proportions of magnesia and silica, 
together with a small quantity of water, apparently in 
a state of chemical combination, a very high tempera- 
ture, approaching white heat, being required to effect 
its complete expulsion, the residue then attaining the 
aforesaid high degree of hardness. The composition 
of talc can be expressed by the symbol H 2 Mg 2 (SiO 3 ) 4 , 
corresponding to 63-52% of silica, 31*72% of magnesia, 
and 4*76% of water. In some varieties of talc, a 
portion (1-5%) of the magnesia is replaced by ferrous 
oxide. Talc is quite unaffected by the action of 
dilute acids, boiling concentrated sulphuric acid being 
required to decompose it, with separation of silica. 

Owing to its low specific gravity and chemical 
indifference, talc is suitable for lightening the shade of 
certain lake pigments. It can also be used as a pig- 
ment by itself, and also as a gloss on wall-paper, for 
mixing with paper pulp, and for various other purposes. 


The mineral known as clay is, in all cases, a product 
of the decomposition of other minerals, mainly felspar. 
This substance is a double silicate of alumina and 
potash, K 2 O.Al 2 O 8 .(SiO 2 ) 6 . Pure kaolin is Al 2 O 3 (SiO 2 ) 2 
+ 2H 2 O, or 46-50% silica, 39-56% alumina, 13-9% 

Clay may be supposed to have been formed by the 
conversion of felspar, under the action of air and water, 
into silicate of alumina, the silicate of potash being 
dissolved out. Being insoluble, the silicate of alumina 
would be transported by the water, in a very fine 


state of division, and finally deposited as a sediment, 
which in course of time became a solid mass. This, 
when again brought into contact with water, forms a 
very plastic pulp which, when dried and baked, forms 
a solid mass, brick, which is no longer affected by 
water. Perfectly pure clay forms a white mass, 
which, under the name of China clay or kaolin, is used 
for making porcelain, and is only occasionally met 
with in large quantities. 

Pure kaolin is characterised by its great chemical 
indifference, being decomposed only by strong alkalis 
and sulphuric acid. At the high temperature of the 
pottery kiln, kaolin sinters to a very compact mass, 
but cannot be fused, except when small quantities are 
subjected to the intense heat of the oxyhydrogen 
flame, whereupon it fuses to a colourless glass of great 

In an impure state, silicate of alumina occurs 
frequently in Nature, and then forms the minerals 
known under the generic names of clay, loam, marl, 
etc. These impure clays contain varying proportions 
of extraneous minerals which produce changes in the 
physical and chemical properties. They are grey, 
blue or yellow in colour, the grey and blue varieties 
mostly containing appreciable quantities of ferrous 
oxide, whilst the yellow kinds contain ferric oxide. 
When fired, all of them become yellow or red, the 
ferrous oxide being transformed into ferric oxide by 
the heat. Some fairly white clays are high in lime, 
which makes them fusible at high temperatures. In 
some very impure kinds, even the comparatively low 
heat of the brick-kiln is sufficient to cause partial 
fusion. For colour-making, the white clays, especially 
kaolin and pipeclay, form a highly important material, 


being procurable at very low prices and fairly easy to 

The white clays aie either used as pigments by 
themselves, or for mixing with other colours of low 
specific gravity. 


The number of yellow earths is large, but most of 
them exhibit a certain similarity in chemical composi- 
tion, the pigmentary principle in the majority being 
either ferric oxide or ferric hydroxide. The former is 
yellow, the latter brown, and the colour of the minerals 
resembles that of the preponderating iron compound. 

Brown Ironstone 

The mineral known as brown ironstone consists of 
ferric hydroxide, and usually forms compact masses, 
no decided crystals having, so far, been observed. 
The lumps have an irregular or earthy fracture, a 
hardness of 5-5*5, and a sp. gr. between 3*40 and 3-95. 
The colour ranges, in the different varieties, from 
yellowish (rusty) brown, through cinnamon to blackish- 
brown. The chemical composition of the pure lumps 
may be expressed by the symbol 2Fe 2 O 3 -j- 3H 2 O; 
but a little manganese oxide and silica is generally 
present even in the pure kinds. 

The chief varieties of this mineral are : 

(a) Fibrous brown iron ore, or brown hematite, 
mostly forming reniform or stalactitic masses. 

(b) Compact brown ironstone, usually in dense 
masses, and not infrequently also appearing in pseudo- 
morphs of other minerals. 

(c) Ochreous brown ironstone. This variety is the 
most important to the colour-maker, for whose purposes 


it is preferably used. It nearly always forms very 
loose, earthy masses, yellow or brown in colour. 

(d) Clay ironstone. This consists of a mixture of 
the above-mentioned varieties with variable propor- 
tions of other minerals, clay being the most common 
ingredient. Nodular iron ore, oolitic, bog and siliceous 
ore belong to this class, as also the minette ores that 
are found in great abundance in Alsace-Lorraine, 
Belgium and Luxemburg, and are classed with the 
oolitic brown ironstones. 

In most cases, the varieties enumerated are found 
together, and are used for the production of iron. 
The ochre constituting the most interesting member 
to the colour-maker often occurs as deposits embedded 
in dense masses of brown ironstone, though in many 
places it is found by itself. 


The following analyses of brown ironstone from different deposits 
will give an idea of the composition of these minerals. 

Ordinary Brown Ironstone 









8. 9. 


Ferric oxide . 


\ j 

73-75 77-54 78-50 


4?'2 5 

Manganese oxide 



2-70 1-95 



| . . 



. . 

33-9 37-88 54-80 






0-15 0-17 0-57 







10-03 0-88 1-15 , 2-50 




0-48 5-08 3-55 2-85 

0-41 0-32 0-50 0-34 



1-25 4-50 0-18 0-90 


0-02 0-38 

Silica . 






33-38 O-O2 ' 0-38 

S0 3 






" i ' 

P 2 5 . 
Sulphur . 






0-06 0-04 Trace 





0-56 0-02 


Loss on incin- \ 
eration / 








7-77 10-55 


Deposits: (i) Hamm; (2) Schmalkalden ; (3) Hiittenberg (Carynthia) ; (4) Styria 
(5) and (9) Bilbao; (6) Algeria; (7) Schwelm (Westphalia); (8) Elbingcrode (Harz) ; 
(10) Pennsylvania. 


Argillaceous Brown Ironstone 

a. b. 







,'. ^ 

Ferric oxide . 

80-76 19-4 










40-90 2I-69 

Manganese oxide 










Zinc oxide 

0-92 1-6 



. . 


2-36 ii-o 







4-95 3-88 









5-59 21-25 



. . 



0-49 0-30 

Silica . 

4-58 48-61 







16-63 14-71 


~ ~ 






1-13 0-48 

S0 3 

. - ' 



Sulphur . 

__ ' 



o-io 0-05 

Loss on incin- \ 
eration / 

1271 ; 9-1 






16-04 28-70 

(a) Oolitic (pea) ore from Elligserbrick (Brunswick) ; (b) from Durlach (Baden) ; 
(c) and (d) Ore from Esslingen ; (e) Oolitic ore from Siptingen (Baden) ; (/) from Adenstedt, 
nr. Pirna (argillaceous) ; (g) Ibid, (calcareous) ; (h) Minette from Esch ; (i) Red minette 
from Dolvaux; (k) Brown minette from Redange. 

Limonite (Bog Iron Ore) 







Ferric oxide 




67-59 7'5 


Manganese oxide 


3-19 3-87 

1-45 178 


P 2 5 . 


0-67 I'I3 

0-18 0-34 


S0 3 . 

3-07 Trace 

0-21 Trace 



6-00 7-00 7-15 



1 6-60 







Lime . 

. . 






Magnesia . 

0'I 5 




Water and \ 
organic acids / 







(i) Limonite from Lausitz ; (2) Limonite from Auer, nr. Moriz- 
burg ; (3 to 6) Swedish limonite. 


Ochre, or yellow Terra di Siena, forms earthy -looking 
masses, fawn, reddish -yellow to brownish-red in 
colour. Whilst not infrequent in Nature, ochre is 
only found in small quantities, as pockets, and not as 
extensive deposits. The discovery of a bed of good 


coloured ochre is, however, always a very valuable 
find, bright natural ochres being somewhat rare, and 
most kinds requiring special preparation before they 
can be used as painters' colours. Owing to the com- 
parative scarcity of good coloured ochres, they are 
often called after the place of origin, such as Thuringian, 
Italian (Siena), English, etc., ochre. 

In nearly every case, ochre is a decomposition product 
of various ferruginous minerals, which has been 
transported by water, often in admixture with other 
minerals, and finally deposited in the places where it 
is now found. Most ochres consist of varying mixtures 
of clay, ferric hydroxide and lime ; and, as a rule, the 
higher the proportion of ferric hydroxide, the deeper 
the colour. Thus, for example, the ferric hydroxide 
may amount, in the dark grades, to 25% of the entire 
mass, whilst in the lighter kinds it may be as low as 
3%. It is very rare that ochre is put on the market 
in its native condition, being mostly subjected to 
chemical treatment enabling a definite shade of colour 
to be obtained. This will be gone into more fully 

Yellow Earth 

Yellow earth is found in many places as compact 
masses, and less frequently as schistous deposits. It 
has a fine earthy fracture, and is mostly devoid of 
lustre, except for a faint shimmer on the surface of 
fracture ; slightly greasy feel ; and a tendency to 
crumble, in water, to a non-plastic powder. It con- 
tains silica, ferric oxide and water in varying pro- 
portions, and the yellow earths from different deposits 
always vary slightly in percentage composition. These 
differences are clearly shown in the following analyses 


of two varieties from the vicinity of Amberg 

(Bavaria) : 

i. ii. 

Silica . . 33- 2 3% 35-io% 

Alumina . . 14-21 I 4'4 

Ferric oxide 


3776 36-80 

13-24 13-60 

When heated, the colour changes gradually to red, 
and the earth becomes extremely hard. There are 
several recognised commercial grades, the price of 
which varies mainly in accordance with the colour 
and fineness. The Amberg variety is specially 
esteemed, the Hungarian and Moravian kinds being 
less valuable. 

The colour not being particularly good, this earth 
is never used for fine work, but is largely employed as 
a yellow wash for houses and as ordinary, distemper. 
It may also be used as an oil paint. 

Red Ochre is a less important, cheap variety of 
ochre, chiefly used in cheap paints and for low-priced 
wall-papers. It occurs in the deposits as clayey 

Terra di Siena 

Terra di Siena is a very pure form of ferric hydroxide. 
When ground, the light to dark brown lumps furnish 
a pale to dark yellow powder, which can be transformed 
into a number of gradations by burning. In spite of 
its handsome colour, this pigment is deficient in cover- 
ing power, in addition to which it darkens when mixed 
with varnish, and dries slowly. 


Apart from the small quantities of native vermilion 
handsome enough for direct use as painters' colours- 


when reduced to powder, the red earths, with practically 
no exception, consist of ferruginous minerals, and it 
is only within a recent period that red painters' colours 
have been prepared from certain chemical waste 
products from manufacturing processes. In all cases, 
however, compounds of iron and oxygen constitute 
the bulk of the red earths. In addition to ferric oxide, 
which is the chief material used for making the im- 
portant red colours, are compounds of ferric oxide and 
water, i. e. ferric hydroxides. The ferric oxide pig- 
ments are among the most important in the entire 
series of earth colours, being on the one hand very 
cheap, and on the other so handsome in colour that 
ferric oxide can be used for the finest paintings. 

Ferric oxide can also be shaded very extensively by 
a fairly simple treatment, so as to furnish a whole 
range of very handsome shades. 

In nature, ferric oxide occurs in numerous varieties 
of one and the same mineral, red iron ore, which is 
also known as hematite, blood stone, raddle, etc. 

Red Ironstone 

Red hematite occurs native as rhombohedral crystals, 
which mostly consist solely of ferric oxide, and may be 
considered as pure oxide for the purposes of the colour - 
maker. The difference between the several varieties 
is due, not to any chemical variation, but entirely to 
changes in physical structure. The varieties with a 
radial, fibrous structure are known as red hematite, 
the colour of which ranges from blood red to dark 
brown and is frequently accompanied by metallic 
lustre. The scaly modification of this mineral forms 
micaceous iron ore, and is usually a deep iron black. 


In the neighbourhood of volcanoes it is frequently 
found as particularly handsome crystals. 

Iron cream (frosty hematite) is the name given to 
a beautiful cherry red variety, which easily rubs off, 
has a greasy feel and is composed of extremely fine 

The so-called raddle occurs in Nature as a readily 
pulverulent earthy mass of ferric oxide contaminated 
more or less with extraneous substances. On account 
of its abundance and low market price, it is largely 
used in painting. 

Although mixed with numerous foreign substances, 
certain clay ironstones, oolitic ironstones and siliceous 
ironstones may be regarded as ferric oxide in the 
sense understood by the colour -maker, all these minerals 
having a deep red to deep brown colour and being 
capable of rinding advantageous employment as 

Ferric oxide is distinguished by two properties 
which render it specially valuable to the colour -maker. 
When combined with water, its colour is no longer 
red, but a handsome brown ; and, on the other hand, 
when heated, the colour passes through brown into 
a permanent dark violet. By suitable treatment of 
such minerals as consist mainly of ferric hydroxide, 
mixtures can be obtained which contain the oxide and 
hydroxide in variable proportions and give a whole 
range of shades between brown and red. 

The preparation of these colours is easy when very 
pure red ironstone is available. The somewhat ex- 
pensive pigment, Indian red, is when pure really 
nothing but a very pure ferric oxide of Indian origin. 
Ferric oxide, however, often contains impurities which 
considerably influence the colour of the product. 


Owing to the fact that large quantities of ferric oxide 
are formed as by-products in certain chemical processes 
which are carried out on a very extensive scale, this 
oxide, which is very pure, can be advantageously used 
for making iron pigments, especially as its application 
for other purposes is very restricted, and it can there- 
fore be had at a very low price. 

The following analyses show the composition of 
a number of red ironstones, Nos. i, 2 and 3 being 
hematite from Froment, or Wetzlar, No 4 from Wetzlar, 
Nos. 5 and 6 hematite from Whitehaven, No. 7 from 
Thuringia, No. 8 from Bohemia, No. 9 from Spain, 
No. 10 from N. America, and No. n from England. 





P 2 5 . 

lime and 








2 -OO 








































T ^ Man- 
Ir0n - ganese. 


3Z. "- 





Loss on 


33-64 o-io 

7-58 8-10 



Trace 0-19 


9 31-38 0-19 

0-06 29-95 





10 62-54 1-93 







ii 62-91 Trace 1-39 0-70 



0-05 o-n 

1 1 

There are certain other minerals closely allied, both 
chemically and mineralogically, to red ironstone, 
namely, the brown hematites or ironstones used in the 


manufacture of iron. Brown hematite consists of 
ferric hydroxide, Fe 2 O 3 H 2 O, and occurs in a variety 
of forms in Nature, the most frequent being pea 
(oolitic) ore, which owes its name to the spherical 
shape of the grains. Some brown hematites are 
decomposition products of other minerals, and contain 
sulphur and phosphorus in addition to ferric hydroxide. 
Like the pure hydroxide, they are biown in colour, 
but differ therefrom considerably in their chemical 
behaviour when heated. This is particularly the case 
with the so-called bog ore, which is mostly found, as 
spongy yellow-brown to black masses, in swamps, and 
owes its origin to the decomposition of various ferrugi- 
nous minerals. It varies greatly in chemical composi- 
tion and occasionally contains up to about 50% of sand. 
The amount of ferric oxide in bog ore varies between 
20 and 60%, and it also contains 7-30% of water, 
up to 4% of P 2 O 5 , small quantities of ferrous oxide 
and manganese hydroxide, together with, in most cases, 
mechanically admixed organic residues. 

The phosphorus content makes bog iron a very 
inferior material for smelting, the resulting iron being 
of low quality. Nevertheless, it can sometimes be 
advantageously used in making earth colours, though 
the products cannot lay much claim to beauty of 


The native earth pigments known by this name 
form masses of the colour of leather to dark brown, 
with a conchoidal fracture and an earthy appearance. 
Bole chiefly consists of iron silicate combined with 
water, some varieties containing small quantities of 
alumina. The composition fluctuates very considerably, 


most varieties containing 41-42% of silica, 20-25% 
of alumina, and 24-25% of water, the remainder 
consisting of ferric oxide. Some kinds, such as 
Oravicza and Sinope bole, contain only 31-32% of 
silica and 17-21% of water. 

Bole is used as a paint for walls, clapboards, etc., 
and is only mentioned here because of its relationship 
to the ferric oxide pigments. 

Alum Sludge 

Large quantities of clarification sludge are produced, 
in alum works, as the sediment from the red liquors. 
This sludge consists mainly of ferric oxide, with small 
quantities of other oxides and sulphuric acid (basic 
ferric sulphate, and would be an entirely worthless 
by-product except for the fact that it can be manu- 
factured into pigments, some of them of great beauty. 

All alum makers should treat this residue and con- 
vert it into pigments, which they could put on the 
market at a low rate, the cost of preparation being 
small. Since the material is chiefly composed of ferric 
oxide, the resulting colours are very similar to those 
obtained from iron ores ; and all shades, from yellow- 
brown, through red, to the darkest brown, are 

Mine Sludge 

The water frequently present in iron mines some- 
times contains large quantities of sediment, which 
consist mainly of iron ochre and can be advantageously 
worked up into pigments. There is scarcely any 
need to mention that all substances containing ferric 
oxide can be used for making any of the pigments 
obtainable from the oxide itself, the only difference 


between the various raw materials being their degree 
of purity, so that it is not always so easy to obtain a 
certain desired shade from a given material in such 
beauty as is furnished by another material, the small 
quantities of impurities associated with the ferric 
oxide having, in many instances, an important influence 
on the colour. 


Only two minerals are known which are capable 
of direct use as blue pigments, viz. vivianite (native 
Prussian blue) and copper carbonate (azurite, ultra- 
marine), and as neither of them is particularly hand- 
some, they are only used for unimportant work. Lapis 
lazuli is no longer employed. 

Azurite, or Ultramarine 

This mineral, which is of frequent occurrence with 
malachite and other cupriferous minerals, forms small 
crystals of a beautiful deep azure blue consisting of 
cupric oxide in combination with carbon dioxide and 
water, expressed by the formula 2CuCO 8 ,Cu(OH) 2 , 
or Cu 3 (OH) 2 (CO 3 ) 2 , and containing 69-19% of cupric 
oxide, 25-58% of CO 2 and 5-23% of water. The 
colour of the powdered mineral is much paler than 
that of the crystals. The pigment, which is used for 
cheap paints, is not particularly stable, and loses much 
of its beauty when applied to plaster. 


This mineral occurs in many places as crystalline 
masses, but also forms earthy deposits, some of which, 
especially in certain bogs, attain considerable thickness. 


The colour is between indigo and blackish blue ; 
and the freshly won mineral often has an unsightly 
whitish appearance, which, however, soon changes 
into the pure blue. The cause of this peculiarity is 
due to the fact that vivianite originally consisted of 
hydrate d ferrous phosphate, which is white, this com- 
pound being transformed, under the influence of the 
air, into the blue ferric phosphate. 

Vivianite contains ferric oxide, phosphoric acid and 
water, but in variable proportions. The original 
composition, expressed by Fe 2 (PO 4 ) 2 + 8H 2 O 2 , corre- 
sponds to 43-03% of ferrous oxide, 28-29% of P 2 O 5 
and 28-68% of water; but, in the air, part of the 
ferrous phosphate is oxidised to basic ferric phosphate, 
so that the content of ferrous oxide may range from 
9-75 to 42-71%, and that of ferric oxide between 1-12 
and 38-20%. Vivianite is also sold as blue ochre, and 
is now seldom used as a painters' colour, owing to the 
introduction of a large number cf artificially prepared 
blues, which are superior to vivianite in colour and are 
cheaply made. However, it can still find application 
in localities where it is obtainable in quantity. 


The green earth pigments comprise green earth 
(Verona green) and malachite. Like the blue earths, 
they cannot lay any particular claim to beauty, but 
they are very cheap, and consequently are largely 
used where low price is the chief consideration. 

Green Earth 

In Nature, green occurs as an entirely non-crystalline 
earthy mass, which is probably a decomposition 


product of augite. It has a close, earthy fracture, 
a colour between seladon and olive green, and a slightly 
greasy appearance. In point of chemical composition 
it consists of silica, alumina, magnesia, sodium, 
potassium, ferrous oxide and water, the usual repre- 
sentative formula being ROSiO 2 H 2 O, in which RO 
symbolises a metallic oxide. 

The colour is due to ferrous oxide ; and if left exposed 
to the air for a long time, or subjected to powerful 
calcination, the great affinity of ferrous oxide for 
oxygen causes the colour to turn red and red-brown. 

Green earth is found in many localities, e. g. Bohemia, 
Hungary, the Tyrol and Cyprus, the finest, however, 
occurring near Verona, on which account it is known as 
Veronese earth. 


The commercial pigment consists of powdered 
malachite, a mineral which usually occurs in compact 
masses of a handsome emerald green colour, though 
isolated lumps exhibit considerable variation in shade, 
some of them being dark green and others very pale. 
In chemical composition, malachite is closely allied to 
azurite, consisting of cupric oxide, carbon dioxide 
and water, and the difference is entirely one of per- 
centage proportions. The formula is CuCO 3 , Cu(OH) 2 , 
or Cu 2 (OH) 2 CO 3 , corresponding to 71-90% of cupric 
oxide, 19-94% of carbon dioxide and 8-16% of water. 

Powdered malachite (even the dark green varieties) 
is always rather light in colour, and for this reason is 
not much used. Furthermore, the mineral is rather 
hard (3-5), and is consequently difficult to grind; in 
addition to which the mineral is fairly expensive, on 
account of its employment as a source of copper, 


particularly fine pieces being also used as ornaments 
or for making works of art. Moreover, like all copper 
compounds, it is very sensitive to the action of sul- 
phuretted hydrogen, and liable to discoloration in 
course of time. 


Numerous minerals are adapted for the manufacture 
of brown pigments. On the basis of chemical com- 
position, they may be classed in two groups; those 
consisting of ferric hydroxide, and those in which the 
brown colour is due to organic substances. 

The first group comprises the minerals which have 
already been mentioned in connection with the red 
earth pigments, bole and brown ochre (umber), Terra 
di Siena, Cologne earth and a number of other earths 
rich in ferric hydroxide belonging to this category. The 
second, or organic group, includes compounds that 
are very rich in carbon and are therefore of a very 
dark colour, the shades ranging from light brown to 
black, e. g. the true umbers and asphaltum. 


As already mentioned, the term " umber " was 
formerly applied to brown varieties of ochre, whereas 
at present it is extended to certain masses of brown- 
coal character, often interspersed with iron ochre and 
sometimes containing manganese. Umber generally 
consists of fairly dense, earthy masses, which are 
dried and ground after crushing and levigation, if 

Valuable varieties are Cappagh brown and Cale- 
donian brown, both with a reddish tinge. 


It is thus evident that " umber " now implies two 
different kinds of materials, organic masses and iron- 
manganese compounds, 'which can also be used as oil 
paints. These umbers can also be extensively shaded 
by burning, the final colour being particularly influenced 
by the amount of manganese compounds present. 

The carbonaceous umbers (Cassel brown, Carbon 
brown) are combustible, and mostly leave behind a 
merely small residue of ash. An important property 
of these umbers is their partial solubility in alkalis, a 
peculiarity which is utilised for the preparation of 
brown wood stains. 

A sphaltum 

Asphaltum forms very friable dark brown to black 
masses, which, in contact with a light, easily ignite 
and burn with a bright, but very smoky, flame, dis- 
engaging a peculiar, " bituminous " smell, and leaving 
only a very small quantity of ash. 

Extensive deposits of asphaltum are found at the 
Dead Sea, the Pitch Lake on the island of Trinidad, 
in Dalmatia, and many other places, where, however, 
it is in an impure condition and frequently contains 
large quantities of sand. In many localities the rock 
is impregnated with asphaltum, which makes it dark 
brown to black in colour and gives rise to a bitu- 
minous odour when rubbed. 

Peat beds sometimes contain pockets of a mass with 
a handsome brown colour and consisting of a mixture 
of humic acids and other organic substances which 
may be ranked with the humin bodies that are always 
formed when organic matter decomposes in presence 
of an insufficient supply of oxygen. These bodies 
are dark coloured, mostly deep brown, rich in carbon, 


and, to some extent, similar to brown coal or peat in 
chemical composition. 

Their high carbon content renders these substances 
very inert towards chemical reagents, and therefore 
particularly adapted for the preparation of painters' 
colours. Genuine Vandyke brown, which is the 
handsomest brown known, is an earth rich in humin 
compounds; and Cassel brown also belongs to this 


The colour of these earths is entirely due to carbon, 
and pure carbon, a certain form of which occurs native, 
is itself used as a pigment. Actually, there are only 
two minerals that require to be mentioned in this 
connection : black schist and graphite. 

Black Schist 

In most cases this is a clay shale, so rich in carbon 
as to appear deep black. In commerce, this mineral 
is also erroneously called "black chalk"; but at 
present it is seldom used as a pigment or drawing- 
material, black chalks being produced far more cheaply 
than the expense of preparing the natural article. 

Grey clay shales are used for making grey earth 
pigments (stone grey, and mineral grey). 


This mineral is found, in a very pure state, in many 
localities, celebrated deposits occurring in England, 
Siberia, Bohemia and Bavaria, whilst North American 
graphite has lately come into prominence. 

Graphite is a modification of pure carbon, and is 


met with in the form of hexagonal (rhombohedral) 
crystals, usually occurring as hexagonal plates with a 
lustrous, iron-black colour. It rubs off easily, and 
readily burns away, leaving a very small amount of 
ash, when subjected to a very high temperature in 
presence of air. 

The principal uses of graphite are as an anticorrosive 
paint for iron, and for making lead pencils. 

As already mentioned, the term " earth colours " 
has been considerably broadened of late. Whereas, 
formerly, it was restricted to colours prepared ex- 
clusively from minerals by a simple treatment, limited 
to crushing, levigation or calcination, it now includes 
the pigments obtainable from large by-products of 
certain chemical processes. This latter class is 
especially important as affording an opportunity of 
utilising products formerly considered worthless and 
whose removal often entailed heavy expense. 

By drawing on these materials the industry of the 
earth colours has greatly enlarged its scope. At 
present, many colours of this kind are on the market, 
and it is to the interest of many manufacturers to 
endeavour to utilise certain waste products in the 
same direction. The advantage of such a course 
hardly needs emphasising ; but, to give only a single 
example, it may be mentioned that the manufacture 
of fuming sulphuric acid from green vitriol, by the old 
process, produces residues which were formerly looked 
upon as quite worthless, and sold at .very low prices, 
but are now worked up, in a number of factories, into 
very handsome and durable pigments. 



THE preparation of the raw materials for the purpose 
of making earth colours is a very important matter, 
because many minerals or pigmentary earths merely 
require mechanical treatment to render them at once 
fit for use. The mechanical preparation differs con- 
siderably, in accordance with the raw material under 
treatment, substances that are found native in a finely 
powdered condition only needing, for the most part, 
to be levigated. 

It rarely happens, however, that the raw material 
occurs in condition for use direct, an example of this 
kind being afforded by the finest clays or ochres. 
Whilst these are found in a state of extremely fine 
powder, they nearly always contain certain quantities 
of sandy ingredients or even large lumps of foreign 
minerals, and therefore require levigating. Sometimes 
they need crushing as well, the small particles cohering 
so strongly that mere treatment with water (levigation) 
is unable to separate them. Mechanical force is there- 
fore necessary, a passage through grooved rollers being 
generally sufficient to crush the lumps; but in some 
cases stamps have to be used. 

When solid materials have to be treated, mechanical 
appliances must always be used, their selection depend- 
ing on the materials in question. Thus, gypsum, for 



example, can be crushed with ordinary rolls or mill 
stones, its degree of hardness being so very low (2) 
that it can be scratched with the finger-nail. 

If, however, the material to be reduced is limestone, 
which belongs to the third degree of the scale of hard- 
ness (can only be scratched with an iron nail), or heavy 
spar (hardness 3-3*5), very powerful stamps or edge- 
runners must be employed to break it down into small 
lumps, which can then be further reduced, without 
any special difficulty, in an ordinary mill. 

It is thus evident that a great variety of mechanical 
appliances are used in the manufacture of earth colours. 
Before going into their construction it is necessary to 
point out that, whatever the mechanical treatment 
employed, a considerable expenditure of mechanical 
force is entailed ; and more power is needed when 
mixtures have to be prepared. It is therefore essential, 
in planning a factory for making earth colours on a 
large scale, to make provision for ample motive power. 

This power may be supplied by a steam engine ; 
but it must not be forgotten that the prime cost and 
running expenses of such an engine are considerable, 
and form an important item in view of the low value 
of most earth colours. Consequently, it is highly 
important to be able to generate motive power as 
cheaply as possible. 

Now, the cheapest and most uniform source of power 
is water; and therefore, wherever the conditions allow 
of the erection of the colour works near a stream or 
river, which can supply the power to run the various 
machinery, the most favourable circumstances will 
have been secured, the power being obtained at mini- 
mum cost, whilst the upkeep of the motor cannot be 
very great. If there is sufficient head for the water 


to be run through a trough over the top of the leviga- 
tion tanks, the conditions will be ideally favourable. 

Wind power costs nothing, once the motor has been 
installed; but unfortunately, one is dependent on 
the weather, and sometimes there is not enough wind, 
for days together, to drive the sails at all, and therefore 
all the operations have to be stopped, including leviga- 
tion, the water for which has to be raised by a windmill 

In districts where the winters are severe, water 
power may also fail and work have to be stopped; 
and consequently, even when water power is the prime 
source of energy, a steam engine must be installed as 
a stand-by, being, of course, only used when the main 
source of power gives out or proves insufficient. 

The machines employed for preparing the raw 
materials in the manufacture of earth colours may be 
divided into the following groups : 

Machines operating entirely by pressure : crushers ; 
machines acting by impact : stamps ; those acting 
by impact and pressure : vertical mills (edge -runners), 
ball mills, centrifugal mills; and, finally, machines 
with a frictional action : grinding mills. Then there 
are the levigating machines, which do not reduce the 
material but separate the coarser particles from the 
finer. The construction of the foregoing machines 
is a matter for the machinery manufacturer rather 
than the maker of earth colours ; but as the business 
of the latter is dependent on them, a short description 
is considered necessary. The selection depends, on 
the one hand, on the nature of the materials to be 
treated, and, on the other, on the size of the works, 
since a manufacturer who has to deal with large quan- 
tities of a given raw material will require different 


machines from those used on a small scale. The sole 
purpose of the following description is to indicate to the 
colour maker the way in which the reduction of the 
raw material can be accomplished. 


Crushers and Breakers. Crushers usually consist 
of grooved iron rollers revolving on horizontal axes. 
One of the rollers is fixed, the other being adjustable 
by screws, in order that lumps of different sizes may 
be treated in one and the same machine, which may 
be employed either to turn out a roughly crushed 
product, or to reduce it to a certain degree of fineness. 

If several pairs of crushing rollers be mounted in 
series, and each set a little closer than its predecessor, 
the material can be reduced progressively from large 
lumps to a fairly fine powder. 

Each pair of rollers is geared together by pinions, 
and is turned in such a way as to draw the material 
in between. If the gear pinions have the same number 
of teeth, the two rollers will revolve at the same speed 
and will then merely crush the material into lumps 
of a size depending on the distance at which the rollers 
are set apart. 

Nevertheless, by simply altering the gear ratio of 
the pinions, the crushing action of the rollers can be 
supplemented by a grinding action, a much finer powder 
being then obtainable than otherwise, the one Droller 
running at a higher speed than the other. 

These crushers differ in strength of construction, 
very strongly built machines being required for dealing 
with large lumps of hard material, whereas substances 
of low crushing strength, such as clay or other earthy 



materials, can be treated in much lighter machines. 
In any case, however, it is advisable to have the machine 
stronger than is absolutely necessary for the work in 
view; for, although the prime cost is thus increased, 
the outlay on repairs will be reduced, and the machines 
can, if necessary, be used on harder material as well. 
The framework supporting the rollers should always 
consist of a strong iron casting; and the machine 

FIG. i. 

should be set up as close as possible to the engine or 
motor, to minimise the loss of power in transmission 
through long shafting, etc. 

Fig. i represents a breaker (made by the Badische 
Maschinenfabrik, Durlach), suitable for the rough 
crushing of clayey materials supplied in large lumps. 
It can, however, also crush shale, lime, chalk, as well 
as hard, sticky masses which would clog up a stone- 

The material fed into this breaker is gripped at once 


by the poweiful projecting teeth, which are connected 
together by sharp-edged ridges, and is crushed in such 
a way that it can be easily reduced still further by a 
succeeding pair of smooth rollers. 

The granulator (Fig. 2), made by the same firm, is 
an example of a machine for crushing harder materials. 

FIG. 2. 

It is similar in construction to a stone-breaker, but 
differs in the movement of the jaws, and combines 
the properties of breaker and grinder, inasmuch as 
it tears the material as well as crushes it. The figure 
shows the machine adapted for direct electric drive. 
If necessary, these granulators can be fitted with 
classifying jig screens. 

Stamps. Stamps or stamping-mills have been used 


from prehistoric times, and were probably employed 
for reducing hard materials long before the introduction 
of grinding-mills. The underlying principle of the 
stamping -mill is very simple. The material to be 
reduced is placed in a trough or mortar, and the ram 
or head, which is of considerable weight, is raised by a 
mechanical device and then allowed to fall freely, from 
a certain height, on to the material underneath, which 
it crushes. The heavier the head and the greater 
the height of fall, the greater the effect produced. As 
a rule, a large number of stamps are mounted together, 
and in such a way that half of them are being lifted 
while the other half are falling. Either a separate 
mortar or trough is arranged under each stamp, or 
else the whole drop into a common trough charged 
with the material under treatment. Sometimes a 
lateral movement is imparted to the material in the 
trough, so as to bring it under the action of all the stamps 
in succession. 

Although the construction of stamping -mills in 
general appears simple, various modifications are 
employed for different purposes. 

As a rule, a single passage through a stamping -mill 
is not sufficient to reduce the material completely 
to the desired fineness, the first product always con- 
taining large and coarse fragments of various sizes, 
as well as fine powder. 

If the latter were left in with the larger pieces for 
the second stamping it would impede the work, and 
the stamping-mill should therefore be provided with 
means for classifying the material discharged from the 
trough, to separate the fine from the coarse and grade 
the latter into sizes. This is usually effected by means 
of a grading-screen. 


Stamping -mills are chiefly used for reducing brittle 
materials. A number of stamps arranged in a row are 
alternately lifted, by means of cams mounted on a 
common shaft, and then let fall on to the material 
lying on a solid plate, or else on a grating through which 

FIG. 3. 

the crushings fall. Fig. 3 is a stamping-mill con- 
structed by H. F. Stollberg, Offenbach. 

These mills are very strongly built, as independent 
units, the frame being of cast-iron and the rams of best 
wrought -iron with interchangeable chill-cast heads. 
In some mills the stamps are rotated during the up- 
stroke, in order to equalise the wear on the heads, and 
also to economise power. 

4 8 


The grating or trough holding the material is per- 
forated with holes, the diameter of which varies with 
the material under treatment and the desired degree 
of fineness in the product. To increase the efficiency 
of the mill, the grating or trough is adapted to move 
while the mill is running, in order to clean itself auto- 

FIG. 4. 

matically. These mills are made in different sizes, 
with 2, 4, 6, or 8 heads. 

Edge-runners. This type of crusher is highly suitable 
for reducing earth colours in large works. The special 
feature of the type is that both stones are mounted 
vertically and turn on a common shaft in the same way 
that a cart wheel does on its axle. These runners are 
particularly useful for reducing clay, chalk and other 
earth colours, which have to be dealt with in large 


quantities. They will also crush fairly large lumps, and 
they can therefore be used for the further reduction of 
materials roughly crushed in a breaker, etc. The 
material may be treated in either the wet or dry state, 
only slight alteration being needed to change from one 
method to the other. 

There are numerous different patterns of edge -runner, 

FIG. 5. 

but all of them can be divided into two groups, viz. : 
mills with stationary troughs, whilst the shaft carrying 
the runners rotates; and those in which the trough 
revolves, and the stones merely turn on the stationary 
horizontal shaft. 

Comparison of the efficiency of the two types has 
shown that the revolving-trough type is the better, 
giving a larger output per unit time with a reduced 
consumption of power. Figs. 4 and 5 show a vertical 
section and plan respectively of this type of edge-runner. 
The trough G is turned by means of a toothed crown 


gearing with the bevel pinion mounted on an over- 
head shaft C driven by a belt pulley N. 

The bearings of the vertical shaft / of the trough 
are situated at L and M. The runners H are loosely 
mounted on the fixed horizontal shaft E and revolve 
in consequence of the friction between them and the 
material in the trough. As the latter revolves, the 
material is continuously pushed aside by the runners, 
and is again brought under them by the action of 

The great advantages afforded by edge-runners, 
in consequence of their simplicity, easy management 
and low wear in comparison with other grinding 
appliances, have led to their reintroduction on a large 
scale. It should, however, be borne in mind that the 
edge-runner mill must be of a pattern suitable to the 
materials it will have to treat. The method of drive 
usually depends on local conditions. The revolving- 
trough type is chiefly useful for mixing, on account 
of the ease with which the materials can be charged. 

The capacity of edge-runner mills depends on the 
nature of the material, the diameter and weight of 
the runners, the speed at which they are run, and also 
on the rate at which the reduced material is discharged 
in order to give place to fresh portions of the charge. 
This is effected by means of two sets of scrapers, the 
individual members of which can be adjusted in any 
direction. Their ploughing action also greatly assists 
the mixing effect. 

Fig. 6 illustrates an edge-runner mill with revolving 
trough and overhead drive; and Fig. 7 one with 
stationary trough and bottom drive; both made by 
the Badische Maschinenfabrik, Durlach. The runners 
are of grey cast-iron, chill -castings or cast -steel being 


used for crushing hard materials. The trough in 
all cases is lined with detachable chill-cast plates. 
Special attention is bestowed on the lubrication of 
all the moving parts, and all the lubricators are easily 

The main shafts of the fixed-trough machines have 

FIG. 6. 

forged cranks, and the metal crank bearings are pro- 
vided with dust caps. All the shaft journals run in 
detachable metal bushes. 

A special advantage attaching to this type is the 
automatic screening device and the returning of the 
screen residue. In some cases, complicated appliances 
are employed to return the coarse residue from the 
screen, bucket elevators, worm conveyors, etc., all 


entailing increased motive power, not inconsiderable 
wear, and a higher prime cost; but in this instance 
the object is achieved, without extra power or wear, 
by very simple means. The dust-proof shell enclosing 
the runners and screen is provided with large doors 
and charging hoppers. 

FIG. 7. 

The motive power required to drive edge-runner 
mills depends on the dimensions of the mill and on the 
class of material to be treated; the larger the mill 
and the coarser the material, the more power needed 
to drive it. 

This type is the more suitable for raw materials 
that are of an earthy character, so that all that is 


FIG. 8. 



necessary is to destroy the cohesion of the particles, 
as is the case, for example, with clay and all earthy 

The wet method of crushing with edge runners is 

FIG. 9. 

particularly suitable as a preliminary to levigation. A 
machine arranged for this purpose is shown in Fig. 8. 
It consists of two sets of edge runners, one with fixed, 
and the other with revolving trough. The material 
is introduced by hand, or by suitable charging mechan- 


ism, into the upper, fixed-trough machine, where it 
is continuously sprinkled with water and kneaded 
by the one runner, and is passed thence to the second 
roller which forces it through the slotted bed into the 
bed of the lower set. The slotted beds of the upper and 

FIG. 10. 

lower set are offset ; and the chief function of the lower 
set, with rotating bed, is to secure intimate admixture 
of the material which, in most cases, is already 
sufficiently reduced. 

Ball Mills. Ball mills are generally used for crushing 
dry materials to fine powder. The mill shown in 
Fig. 9 is a typical form of grinding drum enclosed in 


a dust-proof casing, the latter being provided, at the 
top, with an opening connected to the dust exhaust 
pipe. The discharge outlet at the bottom can be 
closed by a slide. 

The drum is provided with two strong lateral shields 
or cheeks (Fig. 10), one of which carries the inter- 
changeable cross-arm and the charging hopper. Both 
cheeks are lined with detachable chill-cast plates, 
to take up the wear. The bed is formed of heavy 
steel bars (which can be turned round), between which 
are arranged adjustable slits for the discharge of the 
reduced material. Guard sieves are mounted all 
round, and close to, the bed, and interchangeable fine 
screens surround these in turn. The mesh of the fine 
screens determines the fineness of the product, and the 
residue falls down on to a plate which returns it to the 
interior of the drum. The reduction of the charge 
is effected by a number of very hard, forged steel balls 
of various sizes. 

The mill must be run in the direction marked by the 
arrow on the outer shell, so that the residue on the 
screens can be returned to the drum by the plate pro- 
vided for that purpose ; and the prescribed working 
speed must be maintained. The mill must not be 
overloaded. The impact of the balls should be mild, 
but distinctly audible. Overloading reduces the out- 
put. Idle running causes the most wear, since the 
balls then roll directly on the bed, which, of course, 
should be prevented as far as possible. The feed is 
continuous; and, of course, only dry material should 
be introduced. 

When the balls have lost size and weight through 
wear, they must be replaced by a fresh set. 

Pulverisers. Pulverisers are the best form of crusher 


for tough and not over-hard materials. They are 
simple and strong in construction, of high capacity 
with comparatively small consumption of power, 
and furnish a good, uniform product, the size of 

FIG. ii. 

which ranges from fine powder to coarse granules, 
according to the screens used and the class of material 

The crushing is effected by a cross-arm beater, 
composed of four to six radial steel arms on. a divided, 


cast -steel hub, keyed on to the horizontal shaft. The 
arms are hardened, and are adjustably and detach ably 
mounted on the hub. 

The beating action of the arms, which run at high 
speed, forces the material against the studded surface 
of the hardened cheeks of the machine and also against 
the hardened square steel bars forming the periphery, 
the repeated impact of the material on itself, as well 
as against the arms and bars, progressively reducing it 
until small enough to fall through the screen on the 
under half of the casing, into a closed receptacle below. 
The screen mesh varies according to the degree of 
fineness required. 

The peripheral bars are mounted in a very simple 
manner, and in such a way that when one edge of the 
bars is worn, a quarter turn brings a fresh, sharp edge 
into operation, so that all four edges of each bar can 
be utilised. 

To prevent the escape of dust, the machine is pro- 
vided with an air-circulation chamber, which maintains 
the flow of air in continuous circulation, the resulting 
strong draught also drawing the fine material through 
the screen and keeping the meshes open. By this means 
the capacity of the pulveriser is considerably increased. 
The interchange of the crushing organs and screens, 
and also the cleaning of the machine, can be effected 
without difficulty or loss of time. 

The charge is introduced through a feed hopper at the 
side, and may vary, according to the size of the machine, 
from nut size to lumps twice as large as a man's fist. 
If necessary, suitable mechanical feed devices can be 

Disintegrators (Figs. 12 and 13). This type of machine 
is used for reducing medium-hard or soft materials, 


especially where it is desired to obtain a comparatively 
large output of a gritty product. 

In the patterns shown, the main shaft is of steel, 
with dust- and dirt-proof red-brass bearings with 
pad or ring lubrication. The spindle case draws out 
to facilitate cleaning. Mechanical feeding attachments 
can be provided. 

FIG. 12. 

According to local conditions, the disintegrator can 
be mounted either on a brick foundation, with lateral 
discharge outlet into a storage bin, or on a raised grating 
of iron joists. 

If the product is to be conveyed to a distance, it 
is advisable to have a hopper-shaped collector leading 
directly to a worm conveyor or bucket elevator. 

The arrangement shown in Fig. 13, in which the 
disintegrator is mounted on a dust-proof cast-iron 



collector, has been found very suitable for colour works 
of various kinds (aniline, lead, mineral and other 
colours), particularly on account of the suppression 
of dust ; whilst the automatic charging worm greatly 
increases the capacity as compared with charging by 

FIG. 13. 

The effect of levigation is based on the circumstance 
that bodies of greater density than water remain longer 
in suspension in that medium in proportion as the 
fineness of their particles increases. This treatment 
consequently enables the finer portions of a substance 


to be mechanically separated from the coarser. Leviga- 
tion is extensively practised in colour works because 
it furnishes powder of finer grain than can be obtained 
by even the most careful grinding. 

The appliances used for levigation may be of a very 
simple character, consisting only of several tubs or 
tanks, mounted in such a way that the liquid con- 
tained in one can be run off into the one next below. 
With this primitive plant, the material to be levigated 
is stirred up in the water in the uppermost tub and 
left to settle until the coarsest particles may be assumed 
to have settled down, whereupon the turbid water is 
drawn off into another tub, in which it is left to settle 
completely. When the clear liquid has been carefully 
drawn off, a fine sludge is left in the bottom of the tub, 
consisting of the fine particles of material mixed with 

When a particularly fine powder is required, a single 
levigation does not always suffice, but the liquid in 
the second tub must be left to settle for a short 
time only, and then run into a third for complete 

A well-designed levigator for treating large quantities 
of powder is illustrated in Fig. 14. A stirrer R, driven 
by cone gearing, is arranged in a wooden or stone vat 
G. The levigating water enters close to the bottom of 
the vat, through the pipe W. When G is half full of 
water, the stirrer is set running, and the substance to 
be levigated is added. After a while, the water laden 
with the levigated powder begins to run off at A into 
the long narrow trough 7\ provided, at the opposite 
end from A, with a number of perforations through 
which the water runs into the trough TV From this 
it escapes through the perforations into the trough T 3 . 



and thence successively into T 4 and T 5 , finally dis- 
charging into the large tank S. 

The coarsest and heaviest of the water-borne particles 
deposit in the trough 7\, finer particles settling down 
in T 2 , and so on in succession, until the water reaching 
the tank S contains only the very finest of all in sus- 

FIG. 14. 

pension, these taking a long time to settle down to 
the bottom. The deposit in the upper troughs can 
be returned to the vat, whilst that in the lower ones 
will be fine enough to dry as it is. The residue in 
the vat is discharged through Z when the operation 
is finished. 

It will be evident that the fineness of the product 
depends on the number and length of the troughs T, 
the larger these factors the more delicate will be the 


particles remaining in prolonged suspension in the 

Many earth colours require no treatment beyond 
levigation to fit them for use in paints. This is the case 
with, e. g., the white clays ; and certain grades of ferric 
oxide, which occur native in the state of fine powder, 
may also be included in this category. In many cases, 
however, if large quantities of a finely pulverulent 
mineral be stirred up with water and left to stand, 
the deposited solid matter forms such a highly coherent 
mass that it can only be distributed in water with 
difficulty, the fine particles adhering so firmly together 
that it is hardly possible to stir them up again completely 
in the liquid by means of a paddle. 

Nevertheless, this can be easily effected by using 
a special appliance of the kind employed by starch 
manufacturers for a similar purpose, viz. the levigation 
of starch. This apparatus is designed in such a way 
that the pulpy charge of material is gradually and com- 
pletely disseminated in the introduced liquid. 

Fig. 15 shows a device of this kind, consisting of a 
circular vessel provided with a step bearing for a ver- 
tical shaft driven by cone pinions. The lower part 
of the shaft is provided with a thread, on which a nut 
is adapted to travel up and down. By means of rods, 
this nut is connected to a wooden cross-bar provided 
with stiff bristles on its lower face. A horizontal handle 
is attached to the nut. The water is admitted through 
the pipe on the right. 

In working the apparatus, the shaft is rotated and 
the handle held firmly, thus causing the nut and 
attached cross-bar to rise to the limit of its travel. 
The levigating liquid, mixed with the material under 
treatment, is then admitted, until the vessel is full, 


and when the solids have completely subsided, the 
clear liquid is drawn off, and the operation is repeated 
until a thick layer of sediment has accumulated on 
the bottom of the vessel. 

To levigate this, the cross-arm carrying the bristles 
is lowered until it just touches the surface of the deposit, 
and a continuous stream of water is admitted through 

FIG. 15. 

the pipe at the side. The bristles gradually disseminate 
the upper layers of the sediment in the water, which 
becomes turbid and is then drawn off into another 
vessel, cement-lined pits being used in the case of 
large quantities. When- the brushes no longer encounter 
any of the sludge, the cross-arm is lowered sufficiently 
to stir up another layer ; and in this way, large quan- 
tities of solid matter can be distributed in water. If 
the cross-arm is rotated at low enough speed, the 


coarser particles of material keep on settling down again, 
and the collecting vessels will receive only the finest 

In addition to the mechanical separation of coarse 
and fine particles, levigation accomplishes another 
purpose, namely that the prolonged contact of the 
treated material with water dissolves out any admixed 
soluble constituents which might affect the quality 
of the colour, the latter being left in a purified condition. 

For successful levigation it is essential that the charge 
should be in a sufficiently fine condition at the outset. 
Clayey raw materials require no preliminary treatment 
other, perhaps, than passing them through a disinte- 
grator, whereas hard, crystalline substances must 
first be ground in a wet mill, such as an edge-runner 
mill with stationary bed, into which the materials are 
fed with an admixture of water, provision being made 
for keeping the charge under the runners all the 
time. The crushed material is screened previous to 

In the levigation process a few vessels of large size 
are preferable to a number of small ones. The nature 
of the material will determine whether any stirrers 
are required or not, these being unnecessary in the 
case of the pigmentary earths, which naturally remain 
a long time in suspension and therefore do not require 
stirring up. 

The pulpy levigated material is taken out of the tubs, 
etc., drained (if necessary) and dried. The draining 
may be effected in bags, or in large plants filter 
presses or hydro-extractors. In these latter instances, 
pumps will be provided for feeding the sludge direct 
to the presses, and conveyors for delivering the pressed 
material to the drying-plant. 



The levigated colour earths form a stiff pulp con- 
taining a large quantity of water, which can be elimi- 
nated in various ways. Usually, the mass is dried by 
spreading it out thinly on boards and leaving it exposed 
to the air until it has become solid ; or else it is only 
left long enough to acquire the consistence of a thick 
paste, which is then shaped into cones or blocks, which 
are allowed to dry completely in an airy place. If 
the colours are to be sold in the form of powder, the 
dried lumps are crushed. 

To accelerate drying, the pulp may be put through 
a hydro-extractor, or dried in hot-air stoves or rooms. 
As, however, this last method entails special appliances 
and also expenditure, this acceleration is only resorted 
to when rendered necessary by special conditions. 

The Hydro-extractor. When a substance is set in 
rapid rotation, it tends to fly away from the centre at 
which the rotational force is applied. The centrifugal 
force thus coming into action increases with the velocity 
of rotation and with the distance of the substance 
from the axis of rotation. 

The centrifugal hydro-extractor consists, therefore, 
of a vessel in rapid rotation; and if a liquid be intro- 
duced into such vessel, it is projected with considerable 
force against the peripheral walls. If the peripheral 
surface be perforated, the liquid portion of a charge 
consisting of liquid and solid matters will be ejected 
through the perforations, while the solid matter remains 
inside. As a rule, a few minutes' treatment in a hydro- 
extractor is sufficient to separate the water from a 
thin pulp so completely that the solid residue is in an 
almost completely dry state. A hydro-extractor which, 


though of an old pattern, is well adapted for the purposes 
of the colour-maker, is shown in Fig. 16. 

The drum A, which revolves easily on a vertical 
axis, is of metal, and is provided with a large number 
of fine perforations on its peripheral surface. It can 
be rotated at high speed by means of the crank / and 


FIG. 1 6. 

pinions d, e, or by the fast-and-loose pulley a b con- 
nected with a source of power. To prevent any of 
the charge from being projected over the rim of the 
drum, the upper edge is turned over so as to leave 
only a comparatively small opening at the top. The 
lower end of the drum shaft carries a strong steel 
spindle, which must be carefully machined and enable 
the drum to revolve as easily as possible. This is 


essential, because even small machines require a com- 
paratively large amount of motive power which is 
not surprising in view of the high speed at which the 
drum has to revolve in order to perform its functions. 

The drum is enclosed in a casing of somewhat larger 
diameter, which may be of any convenient material. 
The bottom of the casing is preferably tapered slightly 
downward, and is covered, at its lowest part below 
the bearing of the drum with a sieve communicating 
with a pipe through which the ejected liquid is drained 

When a liquid, containing solid matter, is fed into 
the drum, which is already running at high speed, the 
liquid is thrown, by the centrifugal force, against the 
peripheral surface of the drum and escapes through 
the perforations, leaving the solid matter behind. 
Where large crystals are in question, as for instance in 
sugar factories, the centrifugal machine can be employed 
without any additional precautions, the liquid being 
expelled and the crystals being practically dried by 
keeping the machine running a short time longer. 
In the case of the pulp obtained by levigating colours, 
however, this procedure would result in failure, because 
the fine solid particles would be ejected along with the 
liquid and the drum would be left quite empty. 

In this case it is therefore necessary to provide means 
for retaining the solid matter in the drum, and allow 
only the water to escape, with which object the drum 
is lined with a bag of closely woven fabric, open at 
the top and fitting snugly against the inner surface 
of the drum. When the drum is first started, the 
ejected liquid is milky, no fabric being sufficiently close 
to retain all the extremely fine solid particles present. 
In a very short time, however, the liquid will begin 


FIG. 17. 


to run away perfectly clear, this occurring as soon as 
the pores in the fabric have become so far obstructed 
by the projected solids as to allow water alone to pass 
through. The milky water is then returned to the 
feed tank and run slowly into the machine. The water 
is very quickly expelled, and the colour remains in the 
drum as a stiff paste, of sufficient consistence to be 
moulded into lumps of any desired shape. The use 
of the hydro-extractor may be particularly recom- 
mended when ample motive power is available and 
accelerated draining is desirable. 

Fig. 17 illustrates a modern type of hydro-extractor 
with bottom discharge and suspended drum, the shaft 
of which is coupled directly to an electro-motor. 

Filter-presses. Whereas the hydro-extractor is 
only used in particular cases for the purpose of the 
earth-colour manufacturer, the filter-press enjoys more 
extensive application. Every filter-press is composed of 
a number of closely fitting press frames, held together by 
the pressure of a screw. These frames, when assembled, 
form chambers provided with inlet and outlet openings. 
vSuitably shaped and stitched filter -cloths are secured 
inside the chambers, and the sludge to be filtered is 
run into the press from a high-level tank. The water 
passes through the filter -cloths and runs off, whilst 
the colour earth gradually fills the chambers. When 
draining is completed, the press is taken apart and 
emptied. In this way the earths are obtained in the 
form of more or less dry cakes, which are then put 
through further treatment or dried. 

Fig. 1 8 shows a Dehne filter-press suitable for the 
earth-colour manufacturer. Wood internal fittings 
are often used, because wood does not affect the shade 
of the colours ; but, wherever the nature of the materials 


admits, iron presses are to be preferred on account 
of their greater durability and the certainty of the 
joints continuing tight. The finer the grain of the 
levigated colour, the more difficult the expulsion of 
the water; but as a rule, a pressure of 115-195 inches, 
water-gauge, will be sufficient. 

If the sludge be run into the press from a tank at 
sufficient height, two charges can be worked in a day, 
but the cakes will not be as firm as butter of medium 

hardness. It is better to pump the charge into the 
press by means of a special diaphragm pump. The 
drainage is then incomparably quicker, the cakes will 
be formed in about an hour and will also be drier. A 
good deal, however, depends, naturally, on the nature 
of the earth colour. 

If the colour contains acid, alkali or salts, the filter- 
cloths can be washed by flushing the press with water 
under pressure. The cloths are made of specially 
fine cotton fabric. The press-runnings, which are 
never quite clear, are collected in a clarifying tank, 


where they are treated with lime and kieserite, whereby 
gypsum is formed, and the mass is put through a filter- 
press, which retains the solids and leaves the effluent 

Filter-cloths which have become choked by use are 
spread on a table and scrubbed with water, or else 

FIG. 19. 

washed in a special machine (Fig. 19), consisting of a 
rotary drum, with belt drive, the rotation circulating 
the water in the interior trough and enabling it to 
extract the dirt from the cloths. The flow and dis- 
charge of the water are controlled by valves, and the 
water may be warmed by admitting steam into the 
machine. The size of the washer depends on that 
of the filter-cloths. 

From the press, the cakes of colour are conveyed 


to the drying-plant, usually by the aid of automatic 

Drying Appliances. The stiff paste or cakes from 
the hydro-extractor or filter-press can be shaped, but 
require to be dried before they are put on the market. 
Drying is a wearisome operation, the finely divided 
material taking a very long time to dry completely, 
even during the summer months, whilst in winter it 
is almost impossible to get certain colours such as 
ferric oxide colours and levigated clay quite dry in 
the air, the inside of the lumps remaining soft and 
pasty after lying for months. 

The only way in which this troublesome delay in 
the completion of the operation can be overcome is 
by artificial dr}dng; but as the employment of arti- 
ficial heat entails expense, it is necessary to carry on 
the process with the smallest possible outlay, in view 
of the low commercial value of most earth colours. 

Long experience has convinced the author that the 
arrangement of the drying-rooms in many colour works 
is based on entirely wrong principles, and that a great 
portion of the heat furnished by the fuel is wasted. 
For this reason the description of a properly arranged 
drying-room will be welcomed by a number of readers. 

It is a well-known fact that hot air is lighter than 
cold. Consequently, when a room is artificially heated, 
the highest temperature will be found just under the 
roof or ceiling, and articles placed in that part of a 
heated room will dry much faster than those near the 
floor. If the drying-room is heated by an ordinary 
stove, articles placed on a fairly low level will only 
dry very slowly, because the hot air flowing from the 
stove tends to ascend. 

In order, therefore, to utilise the entire space of the 


drying-room, it is necessary to place the heating appar- 
atus in such a position that the whole of the room will 
be warmed as uniformly as possible. The stove should 
therefore be situated in a chamber underneath the 
drying-room proper. 

Because air that is already saturated with moisture 
cannot take up any further quantity, care must be 
taken to remove the damp air continuously from the 
drying-room, and to replace it by dry air. This may 
be effected by suitably designed ventilation, on the 
lines shown in Fig. 20, which represents a drying-room 
arranged in such a way as to provide for all the above- 
mentioned contingencies, and ensure continuous drying. 

The heating apparatus is located in the cellar, and 
consists preferably of a slow-combustion stove com- 
prising a cast-iron cylinder, with an air inlet A (with 
sliding regulator T), for the air of combustion, and 
a shoot F at the top, through which the stove is fed 
with fuel preferably coke, on account of its great 
heating power. 

The stove is surrounded by an iron or brick shell M, 
having two flues R and Ri leading to the chambers 
I and II, where they terminate in register cowls K, 
which can be adjusted, by turning the handles h, so 
that when the slots o in K coincide with corresponding 
slots in the end of the pipe, the maximum amount of 
hot air from the stove is delivered into the drying- 
chambers; and, by suitably adjusting the cowls and 
the draught through the fire-door T, it is possible to 
regulate the temperature of the chambers to within 
one degree of the thermometer scale. When only one 
of the drying-chambers is required to be heated, the 
register in the other is closed, and the whole of the hot 
air is delivered to the first one. With this arrangement 


none of the heat is wasted, and the contents of one 
chamber can be dried while those of the other are being 
removed and replaced. 

The moisture-laden air from the drying-chambers 
can be led direct into the stove chimney. When coke 

FIG. 20. 

is used, the flue gases consist almost entirely of carbon 
dioxide. If the vent pipes are led from the top of the 
drying-chambers into the chimney, the hot gases ascend- 
ing the latter induce a strong draught in the chambers 
and carry off the moist air into the open. These pipes, 
also, are fitted with registers, which, when suitably 


adjusted, assist in the maintenance of a uniform drying 

The colours to be dried are spread on trays laid on 
suitable racks in the drying-chambers ; and, by carefully 
planning out the available space, a very large quantity 
of colour can be quickly and completely dried in a 
comparatively small plant. The cost of the fuel is so 
small as to be more than counterbalanced by the saving 
of time. 

The heating arrangements in drying-rooms are 
capable of improvement in many respects, especially 
where steam is at disposal ; and in such cases, it is 
better to substitute steam heating for a fire. It will 
then be necessary to put in a good fan, or other device, 
to ensure the removal of the moist air. An excessive 
room temperature above, say, 50 C. (122 F.) 
is not only superfluous, but in many cases injurious, 
because, apart from the fact that some colours change 
in shade when over -warmed, an unduly high tempera- 
ture causes the surface layers to dry very quickly and 
form a crust which prevents the escape of water vapour 
from the interior of the material. 

Another form of drying-plant for earth colours is 
the drying-floor, a large room with a rammed concrete 
or stone floor, intersected with brick flues (about one 
foot square), covered with iron or concrete slabs and 
conveying hot flue gases from a furnace. These floors 
are particularly suitable where there is a possibility 
of utilising an existing supply of hot flue gases. 

Drying-tunnels are specially adapted where large 
amounts of material have to be dried. The tunnels 
are built of brick and provided with a rail track on 
which the trucks carrying a series of trays laden with 
colour are run. As the trucks move slowly forward, 


they are met by a current of hot air which dries the 
charge. The tunnel is kept filled with laden trucks, 
each fresh one introduced pushing a finished one out 
at the further end. 

In many cases, drying troughs are also useful. These 
are long, semicircular, jacketed troughs of boiler plate, 
hot air or steam being passed through the jacket space. 
A worm conveyor keeps the contents moved forward, 
turned over and mixed to facilitate drying. 

Mention may finally be made of vacuum drying- 
cupboards, which are heated, air-tight chambers, for 
the material, in which the air is partially exhausted, 
thus increasing the rate of evaporation of the water 
and causing the materials to dry quickly at a much 
lower temperature than otherwise. 


The distributing and covering power of the earth 
colours depends apart from their special properties 
on the fineness of their particles. For this reason, 
all the means adopted for the purpose of pulverisation 
are of particular interest. The most important crush- 
ing and powdering devices have already been described, 
and may be referred to, all that needs mention in 
addition being the fact that stone mills also are used 
for fine grinding. 

The ground products, however, are not entirely 
homogeneous, always containing, in addition to the 
very finest particles, those of a coarser nature which 
must be removed by sifting. 

Sifting machines are essentially sieves through which 
the colour is passed. The sieves are made of wire 
gauze or bolting-cloth, stretched on prismatic frames 


which are rotated (centrifugal sieves), or superposed 
on the flat and reciprocated. In centrifugal sieves, 
the material is projected against the sieve, and the 
whole apparatus is in a state of vibration, or else beaters 
are provided to keep the fine orifices in the sieve from 
choking up. 

Nowadays there are numerous types of sifting 

FIG. 21. 

devices, none of which, however, can be considered as 
the best for all purposes, since each type of earth colour 
behaves differently and requires special treatment. 
The proportion of moisture in the material, also, has 
an important influence on the method of treatment 

A typical flat sif ting-machine, with eccentric jig 
motion, is illustrated in Fig. 21. The machine is 
fed through a hopper provided with feed rollers, the 


rate of feed being adjustable. The screened product 
is discharged through a shoot at one side of the machine, 
and the residue at the opposite side, into boxes, etc., 
placed underneath. 

For materials that give off a large amount of dust, 
the machine can be enclosed in a dust-proof casing, 
in which event the product and residue are delivered 

FIG. 22. 

into drawers. The machine is easily cleaned and the 
sieves quickly changed, and is well adapted for dealing 
with a succession of different materials. The hopper 
can be fitted with a pair of adjustable crushing 

Fig. 22 is a drum sifter, which is fed by means of a 
hopper and worm ; and the drum can be covered with 
wire or silk gauze. The sifted product falls into a 
worm conveyor in the bottom of the casing and is 



discharged at the side. This may be replaced by a 
series of mouths for discharging direct into bags, or the 
machine can be adapted to deliver into an elevator, 
worm conveyor or other means of transport to a 

The screenings are discharged through a shoot at 
the back of the machine, and can be handled in various 

FIG. 23. 

ways. A beater is provided to clear the drum and 
increase the output. 

Fig. 23 illustrates a centrifugal sifting-machine for 
producing very fine powder in large quantities without 
any escape of dust. It contains a screening drum, 
the frames of which are detachable and facilitate chang- 
ing the sieves. A beater revolving inside the drum 
projects the powder against the sieves, such portions 
as pass through being taken up and discharged by a 
worm conveyor; this, however, can be replaced by a 
bagging device, etc. 



Colour earths are sometimes calcined at a high tem- 
perature in order to modify their structure and shade, 
the operation being accompanied, in some cases, by 
the destruction of organic admixtures and the expulsion 
of volatile constituents. 

An important feature of calcining is that it improves 
the covering power of many colours, especially heavy 
spar and certain ferric oxide pigments. This alteration 
is probably due to the heat causing the finest particles 
to cohere, and also to the expulsion of chemically-com- 
bined water, etc. 

The change of shade, which is often dependent on 
the degree and duration of the heating, is probably 
also connected with cohesion ; but in many instances 
it is attributable to chemical modifications produced 
by the treatment ; ferric hydroxide, for example, losing 
its water of hyd ration when heated and becoming 
transformed into ferric oxide. 

The details of the calcination process vary with the 
nature of the material, and will therefore be described, 
together with the appliances used, when we deal with 
the colours which require to be put through this 


It is very important that the maker of earth colours 
should always be able to turn out his products uniform 
in shade, and since the raw materials are liable to vary 
in character, and the composition of the earths from 
one and the same deposit is not invariable, the desired 
shade has to be obtained by mixing. For this purpose, 


standard samples must be prepared, for comparison 
in matching. 

Mixing is a highly important operation, on the proper 
performance of which oftentimes depends the sale of 
certain colours and the reputation of the maker. It 
may be effected in various ways, such as shovelling 
the ingredients together or by combining the work 
with grinding in edge-runner mills, ball mills, etc. 
Another method is the mixing barrel shown in Fig. 24, 

FIG. 24. 

a strong cask mounted on an axial shaft driven by a 
motor, etc. The barrel is filled about two-thirds 
full of the materials to be mixed, and, after closing 
the feed door, is slowly rotated, since, if run at excessive 
speed, the contents are merely projected against the 
sides of the barrel by centrifugal force, and it can then 
be turned for hours without result. The mixing effect 
can be considerably increased by mounting the barrel 
so that the shaft is offset from the longitudinal axis 
of the barrel by an angle of about 30, the contents 
being then moved from side to side at each revolution 


and thus more intimately intermixed by the twofold 

In addition to such home-made appliances, there 
are mixing-machines of the type illustrated in Fig. 25, 
the body of which is fitted with a distributing worm 
at the top, and a pair of rollers at the bottom. Below 
the rollers, which are covered by plates that can be 

FIG. 25. 

adjusted at a convenient angle, is a worm conveyor 
delivering into an elevator, outside the machine casing, 
which connects the two worms. One or more discharg- 
ing-doors, according to the size of the machine, are 
provided under the worm conveyor at the end next 
the elevator. The feed hopper can be arranged on 
the elevator or on top of the machine, according to 
local conditions. 

In working this machine, the elevator and distributing 
worm are started and the full charge is fed into the 


hopper. When it has all passed through the distributor 
and is lodged on the sloping plates and bottom rollers, 
the latter and the worm conveyor are set in motion, 
the material being then carried through by the rotation 
of the rollers and dropping on to the conveyor, which 
delivers it to the elevator, to be returned to the dis- 
tributor. In this way the charge is kept in continuous 
circulation, and the finely divided particles are repeat- 
edly intermingled, a uniform mixture being obtained. 
The effect is heightened by the grinding action of the 
rollers as the material passes between them. 

The serial order of the various ingredients, their 
physical condition (granular or powder), and their 
density, are all immaterial, the mixing being effected 
so intimately that when, for example, a colour is shaded 
with aniline dyes, the ingredients are so completely 
blended in less than an hour that even the smallest 
sample then taken will perfectly represent the bulk. 

These machines are made in various sizes, are entirely 
automatic, both in charging, discharging and mixing, 
and are quite dust-proof, the consumption of power 
being also small. If necessary, they can be combined 
with a crusher or sifter feeding direct into the hopper. 

A simple means of ascertaining whether the mixing 
is completed, and one that can also be employed for 
judging the character of ground materials, consists 
in placing a sample of the product on a sheet of white 
paper and spreading it out, under gentle pressure, 
with a steel or horn spatula. No irregularities, streaks, 
spots or granules should then be discernible either by 
the unaided eye or under a magnifier. 

Improving, which consists in staining earth colours 
with other (usually organic) colouring agents, to improve 
the shade, is an operation which is generally resorted 


to only in case of need, because it means extra expense, 
and is of no value unless light-proof colours are used. 
No permanent effect can be obtained by merely mixing- 
in coal-tar dyes at random. In addition to certain 
organic dyestuffs, artificially prepared mineral colours 
and colour lakes artificial preparations of an organic 
dyestuff with an inorganic substratum are also used 
for improving. 

Another way of improving earth colours is by pre- 
cipitating certain coal-tar dyes on them, in presence 
of a fixing agent. Of course the dyes used must not 
only be fast to light, but also inert towards the sub- 
stratum and to any other ingredient, such as lime, that 
is subsequently added to the earth colours. 

The following dyestuffs (Hochst) are suitable for 
direct precipitation on siliceous colours (green earths, 
clay, ochres, etc.). 

Auramine, cone. O, I, II ; new phosphine extra ; 
chrysoidine A cryst., B cryst., C extra; Vesuvine 
(all marks) ; cachou brown D, G ; dark brown M, MB ; 
safranine G, GS cone., B cone.; rhodamine O extra, 
B, B extra ; fuchsine (all marks) ; fuchsine acetate ; 
cerise G, R; grenadine O, R, RR; maroon O extra; 
new fuchsine O, P ; methylene violet (all marks) ; 
peacock blue P ; Victoria blue B, R ; thionine blue GO ; 
methylene blue (all marks) ; malachite green (all 
marks) ; brilliant green (all marks) ; coal black O, I, II. 


The colour pulp can be made into tablets by moulding 
it in dry boxes divided into a number of compartments. 
The colour shrinks in drying, and the tablets will then 
easily fall out of the moulds. Cones are obtained by 


placing the pulp in a box, the bottom of which is per- 
forated with numerous holes of uniform size, the box 
being then tapped against the surface of a stone table. 
At each stroke, a certain amount of colour is forced, 
in the shape of small cones, through the perforations, 
on to a sheet of paper underneath. The cones are 
then dried. 

Some colours are moulded into blocks by forcing 
the partly dried paste into suitable moulds preferably 
of metal, so that they may be engraved with the maker's 
name, or other imprint and left to dry slowly and 
without cracking. The cakes may be prevented from 
crumbling by incorporating a small quantity of adhesive, 
such as a weak solution of dextrin, with the water in 
which the colour is suspended. 



THE white earth colours are important for the 
purposes of the colour-maker, because not only are 
they used by themselves as paints, but also serve in 
the production of light shades of other colours. 

The white colours containing clay or lime are the 
most abundant and important of all, and will therefore 
be described first. The lime colours comprise caustic 
lime, carbonate of lime (chalk or powdered limestone), 
gypsum and bone ash. 


Though this product is not used direct as a painters' 
colour, it is employed in the preparation of compounds 
that are so used. It is made on a large scale for the 
preparation of mortar, and there is therefore no need 
for the colour-maker to manufacture it himself, since 
it can always be bought from a lime-burner. It must 
be borne in mind, however, that lime for the colour- 
maker's purposes must possess certain properties, 
failing which it is of no use to him. What these 
properties are and how the product is made will now 
be briefly described. 

When carbonate of lime, i. e. native limestone, is 



exposed to strong heat it parts with carbon dioxide 
and is transformed into burnt or caustic lime. 

CaCO 3 CaO + CO 2 

Carbonate Caustic Carbon 

of lime. lime. dioxide. 

The limestone is burned either in kilns of very simple 
construction, or else in more complicated furnaces 
in which a continuous process is maintained. The 
ordinary limekiln, which can be found in many parts 
of the country, consists merely of four walls, with a 
door in the front one for the introduction of the fuel. 
Kilns of this kind are usually set up in the vicinity of 
the limestone deposits, and are abandoned when they 
get worn out. 

The limestone is broken to lumps of fairly even 
size, about as large as a man's head, and these are piled 
up in a domed heap in the kiln, sufficient space being 
left between the lumps for the passage of the flame. 
A fire is then lighted under the pile, pine wood being 
mostly used for this purpose on account of its high 
content of resin, which gives a very strong flame. The 
fire is kept up until the top of the pile has become 
white hot, and only a blue, smokeless flame is visible. 
The appearance of this denotes that the burning is 
ended, the fire being then allowed to die out and the 
lumps left until cool enough to be taken out of the kiln. 

This operation is performed with great care, par- 
ticular importance being attached to preserving the 
lumps as intact as possible and preventing the formation 
of dust, which is of little value. The lime made in this 
way is endowed with properties that render it valuable 
for the purposes of the colour-manufacturer; but, on 
the other hand, possesses certain disadvantages. 


Owing to the use of wood as fuel, the caustic lime 
obtained in this way is usually a very pure white, 
because the burning is continued until the whole mass 
is glowing and the firewood has been completely 
consumed. If this is not the case, the burnt lime is 
greyish in colour, from the finely divided particles of 
carbon, which, of course, spoils the lime for colour- 
making. The defects existing in lime burned in the 
above type of kiln originate in the irregular character 
of the product. It will be evident that the limestone 
lumps that are nearest the fire will be far more strongly 
heated than those in the upper part of the dome ; and 
when calcined lime is kept incandescent for a long time, 
it becomes so compact in texture that it quenches with 
great difficulty when brought in contact with water. 
This condition is known as " dead burnt," and such 
lime is of little value. 

The lumps at the top of the pile are least exposed 
to the heat, and very often still contain carbonate, as 
is shown by the effervescence produced on treatment 
with an acid. Such lime is imperfectly burnt, and 
the lumps frequently still exhibit the crystalline 
structure of limestone when broken. They quench 
rapidly, but when mixed with a little extra water, the 
mass is no longer of the buttery consistency typical 
of caustic lime, but contains gritty portions consisting 
of unaltered limestone. 

Owing to the defects of dead burning on the one 
hand and insufficient calcining on the other, colour- 
makers now prefer lime that has been burned in con- 
tinuous kilns, because, when properly made, such lime 
is very uniform in character, and is also cheaper than 
that burned with such an expensive fuel as wood. In 
consequence of the greater capacity of the continuous 


kiln, and the more uniform character of the product, 
the old-fashioned kilns are more and more falling into 

The arrangement of the continuous kiln is very 
simple. The kiln consists of a fairly high shaft, open 
at the top, and provided at the bottom with a small 
hole for the removal of the burnt lime. A coal fire is 
lighted, and as soon as the kiln is heated up, alternate 
layers of limestone and sufficient coal for burning it 
are introduced. The burnt lime sinks to the bottom 
of the shaft and is pulled out, with iron hooks, from 
time to time. 

Given the right proportions of coal and limestone, 
the lime made in these kilns is burnt to just the right 
degree, and is excellent for builders' use. In many 
cases, however, it is less valuable to the colour-maker, 
and in some quite useless. For example, when the 
coal is not completely consumed, carbon, even though 
only a very small quantity, is deposited on the lime, 
and the burnt lime, instead of being a brilliant white, 
as it should be, is grey ; and colour made therefrom is 
also greyish white and will spoil the shade of other 
colours with which it is mixed. 

The chemical composition of the original limestone 
also has an influence on the character of the burnt 
lime. Limestone consisting entirely of carbon dioxide 
and lime is so rare that sufficient is never available 
for making burnt lime on a large scale. Even the 
purest limestone found native in large quantities 
namely marble is not pure carbonate of lime, but 
contains a certain proportion of extraneous substances. 
At the same time it is too expensive to use for technical 

The ordinary impurities present in limestone are 


ferrous oxide, ferric oxide, magnesia and organic 
matter. The presence of ferrous oxide can usually be 
detected by the greenish tinge of the raw limestone, 
and the reddish cast of the burnt product. Ferric 
oxide is revealed by its reddish colour, in both the 
limestone and burnt lime. 

Magnesia, which is present, for example, in dolomitic 
limestone, cannot be detected by the colour, either 
before or after burning, this oxide being itself perfectly 
white ; but its presence is a drawback because if in 
large quantity it makes the lime very difficult to quench, 
and such lime is never of a fatty character. 

Organic matter betrays itself by the colour, the 
lime being dark tinted, varying from grey to black. 
Black limestones usually contain carbon in an extremely 
fine state of division, and are quite useless to the colour- 
maker owing to the impossibility of completely burning 
off this contained carbon, which always imparts a 
greyish tinge to the burnt lime. The behaviour of 
limestones in this respect varies, however, considerably, 
and can only be ascertained with certainty by a trial 
burning. Many that are rather dark in colour will, 
nevertheless, burn perfectly white, whereas others, 
much lighter in shade, always give a product that is 
not quite pure in tone. This divergent behaviour seems 
to have some connection with the chemical composition 
of the organic matter in question. If it consists of 
coal, or substances analogous thereto, no really pure 
white lime can be obtained from a light grey limestone, 
it being impossible to burn off the finely divided carbon 

In addition to making a trial burning with a fairly 
large sample of material, the behaviour of a limestone 
towards hydrochloric acid will afford some information 


as to the nature of the grey colouring matter. If the 
limestone dissolves completely when suffused with the 
acid, the indications are favourable for its usefulness 
to the colour-maker. If, on the contrary, a black 
residue is left, the coloration is due to finely divided 
carbon, and there is then little prospect of the material 
furnishing a suitable product. In any event, a trial 
burning is the most reliable guide. In addition to 
carbon, the presence of any large proportion of ferric 
or ferrous oxide is objectionable, since, in either case, 
the product will be tinged red with ferric oxide, into 
which the ferrous oxide is transformed at calcination 

In addition to comparing the colour of the product 
with a standard sample, the suitability of a burnt lime 
for colour-making can be tested by quenching. If a 
lump about the size of the fist be placed in a large 
porcelain basin and suffused with a small quantity of 
water, preferably poured in a thin stream, the lime, 
if properly burned, will continue to absorb the water 
for a considerable time, like a sponge, and will very 
soon give evidence of a brisk reaction by increasing 
in bulk and generating such an amount of heat as to 
cause the immediate evaporation of a few drops of 
water allowed to fall on the surface of the mass. 
Finally, the entire lump will crumble down to a very 
delicate, voluminous powder, consisting of slaked lime 
(calcium hydroxide). 

This chemical reaction is expressed by the 
equation : 

CaO + H 2 O Ca(OH) 2 

Lime Water Calcium hydroxide. 

When the amount of water added to burnt lime is 


no more than sufficient to effect its transformation into 
hydroxide, this latter, as already stated, forms a deli- 
cate white powder. The addition of more water results 
in the formation of a homogeneous pulp, of a peculiar 
fatty character. Since this fatty appearance is only 
possessed by pure lime, it is a criterion of high quality 
in burnt lime, and contrasts strongly with that of the 
less valued poor (or lean) lime. 

Calcium hydroxide acts as an extremely powerful 
base, and therefore must not be mixed with colours 
that are sensitive to the action of strong bases. As a 
matter of fact, its direct use in painting is very small. 
Of course, a thin milk of lime is used for whitewashing 
walls, etc. ; and if any colouring ingredients are added 
they must be such e. g. ochres as are not affected 
by the lime. Nevertheless, quick and slaked lime are 
very important in colour-making, as forming the 
originating material for the preparation of a number 
of colours. 

When slaked lime is mixed with sufficient water to 
form a stiff pulp, and is left exposed to the air for some 
time, a change will be observed to take place, the mass 
solidifying gradually (commencing on the outside) and 
finally crumbling to a soft white powder. This change 
is due to chemical action, the lime having a great 
affinity for carbon dioxide, which it readily takes up 
from the atmosphere a fact which explains the 
solidification mentioned. It would be erroneous to 
assume that the lime is again completely converted 
into calcium carbonate in this way ; for, though such 
conversion does ultimately take place, it requires a 
very long time for completion. 

The resulting compound is, actually, a double com- 
pound of calcium oxide and carbonate. Although this 


compound has fairly strong basic properties, they are, 
nevertheless, far weaker than those of caustic lime, 
being partly neutralised by the carbon dioxide absorbed. 
If the superficial area of the slaked lime be increased 
by spreading it out thinly, so as to offer greater 
opportunity for the action of carbon dioxide, the 
formation of the double compound in question will be 
greatly accelerated. 

This double compound is prepared artificially in 
special works, and the resulting colours are put on the 
market under various names. They, too, must not be 
mixed with colours that are sensitive to alkali, and on 
this account they cannot be used in fine paints. If 
applied as a white priming to the walls of rooms, care 
must be taken to cover the coating with some substance 
that will protect the topping colour from the action of 
the lime. For this purpose, painters use a wash of 
milk, soap and water, etc. 

An important property of lime is its behaviour 
towards casein, the substance forming the curd of 
milk. With this body it combines to form a mass 
which sets hard -and is highly resistant, viz. calcium 
caseate, and is formed when limewash is stirred up 
with milk or freshly precipitated casein. Weatherproof 
distempers for outside use are prepared in this manner. 


The preparation frequently met with in commerce 
under this name is nothing more than a burnt lime of 
great purity. It is prepared in the coastal districts 
by burning oyster shells, the resulting burnt lime being 
easily transformed into a fine powder, the pure white 
colour of which is due to the absence of iron. It is 
used in the same way as pure burnt lime, and is mainly 


of interest in seaside towns where oyster shells are often 
accumulated. It may be pointed out that the name 
pearl white is often applied also to pure white grades of 
white lead. 


This colour is prepared from any kind of burnt lime 
that is sufficiently pure ; that is, free from ferric oxide. 
The method of preparation is simple, requiring no 
special apparatus, and can therefore be carried out 
wherever suitable lime is available. 

Operations are commenced by carefully slaking 
well-burnt lime with water, a sufficient excess of which 
is added to produce a fairly thick pulp. To accelerate 
the absorption of carbon dioxide, the mass is exposed 
to the air in thin layers, by spreading it out on boards, 
so as to present a large surface to the air. As soon as 
the pulpy character has disappeared, the mass is 
detached from the boards, and is pressed and kneaded, 
with wooden paddles, into prismatic cakes which are 
left exposed to the air being, of course, protected 
from the wet until the absorption of carbon dioxide 
is complete a condition that can be recognised by 
the earthy character of the product. The cakes are 
then dried, an operation entailing great care, since 
lightness is a sign of good quality, whereas a damp 
product is very heavy. 

In forming the cakes they must not be touched by 
the bare hands, because the lime is so caustic that it 
would soon destroy the skin. The foregoing method of 
manufacture is capable of many improvements, which 
can be introduced without adding much to the cost of 
If the lime is formed into large blocks, it will evidently 


take a long time for the mass to acquire, all through, 
the earthy character indicating combination with 
carbon dioxide. This drawback can be easily remedied 
by forming the mass into small cakes, which will 
become ripe, owing to their larger surface, much sooner 
than the bigger blocks. 

A very good plan to adopt in moulding is to form the 
burnt lime into a stiff paste with water, preferably by 
adding enough water to make a viscous mass, and 
leaving this in a lime-pit for several weeks, the prolonged 
storage enabling the lime to acquire the already 
mentioned fatty character, and at the same time to 
become highly plastic. Lime treated in this way can 
be forced through a nozzle, forming a cylindrical rope, 
which can be cut by a knife into convenient lengths 
and left on boards for a few days until they have 
become firm enough to stand up without breaking. 
Cylinders made in this manner, with a length of about 
four inches and a diameter of two inches, will absorb 
carbon dioxide very quickly. 

The absorption can be still further accelerated by 
setting up the cylinders in an atmosphere highly 
charged with the gas, for instance in the vicinity of a 
manure pit, as they will then avidly take up the carbon 
dioxide abundantly liberated from the rotting manure. 
Similar acceleration will take place if the boards 
carrying the cylinders are placed in a stable, or in a 
room where wash for making spirits is fermenting, 
because large quantities of carbon dioxide are liberated 
in both places. 

Working the caustic mass by hand is accompanied 
by so many inconveniences that it seems highly 
desirable to employ some mechanical moulding device 
which will render contact with the wet lime entirely 



superfluous. It may be pointed out that such a device 
can also be advantageously used for moulding all earth 
colours in paste or pulp form, and in particular for 
shaping ferric oxide colours into rods or small cylinders. 
Such a machine (Fig. 26) is composed of a rectangular 
box with semi-cylindrical bottom, a detachable shaft 
carrying a sheet-metal worm being arranged in the box 
so that the worm is in contact with the rounded bottom 
and is continued into the cylindrical extension of the 

FIG. 26. 

box. This extension terminates in a hollow cone, to 
the mouth of which nozzles of varying aperture (square, 
rectangular or round) can be attached. A knife, 
operated by hand or mechanical means, enables the 
extruded soft mass to be cut into convenient lengths, 
which drop on to a series of easy running rollers in 
front of the nozzle, and are thereby delivered to an 
endless-belt conveyor from which they can be trans- 
ferred to the drying-boards. 

When the box has been charged with the lime pulp 
and the worm is rotated, the latter forces the soft mass 
into the cone and extrudes it through the nozzle, so 


that, as long as there is any material in the box, it is 
discharged as a continuous rope, of square, rectangular 
or cylindrical section, on to the guide -rollers, where it 
can be cut off into lengths by the knife. 

A fundamental condition for the preparation of a 
good Vienna white is the employment of pure raw 
material, which must be free from ferric oxide or 
earthy impurities, and fully burned. An excellent 
material for this purpose is calcined mussel shells, 
which furnish a loose, and at the same time very pure, 
lime, and are very largely used for lime -burning in 
places such as Holland, where they are available in 
large quantities. 

Vienna white is not much used as a paint colour, 
owing to its powerful alkaline properties which have 
a destructive effect on many colours. It is, however, 
largely employed as a polishing agent, for which purpose 
it is powdered and is put on the market mostly in 
bottles as Vienna lime. Its very handsome white 
colour and low price render it particularly suitable for 
coarse painting, for example as a prime coating for 
painted interior walls. To guard against the danger 
of the painted decoration being destroyed by the 
alkaline nature of the white, it is advisable to coat the 
dried ground with alum solution, the alumina of which 
combines with the lime to form an insoluble compound 
to which organic colours adhere well. The sulphuric 
acid also enters into combination with the lime, the 
resulting gypsum having no effect on the paints 
subsequently applied. 


The name chalk is used for a number of commercial 
substances which differ considerably in both the 


mineralogical and chemical sense. French chalk, for 
instance, is a mineral belonging to the steatite group 
and, apart from its name, has nothing in common with 
true chalk, except the white colour, and even this 
differs altogether from that of chalk properly so called. 
It is therefore necessary, in the interests of proper 
nomenclature, to differentiate the various kinds of 
chalk, commencing with the mineral known by that 
name to the chemist and mineralogist. 

In chemical composition, true chalk is calcium 
carbonate, but of a fossil character, for if chalk dust 
be examined under a high-power microscope, it will be 
seen to consist of the shells of minute animals, and is 
therefore to be regarded as fossil. The organic matter 
of the animals has long disappeared, leaving the 
inorganic material, a very pure calcium carbonate, 

Such progress has been made that the zoological 
status of the animals which inhabited the shells many 
thousands of which are present in a lump of chalk- 
has been identified ; and it is known that these animals 
were of marine type. Fig. 27 shows the appearance 
of the animal remains in Meudon chalk when highly 
magnified, the upper half being viewed by transmitted 
light and the lower by reflected light. 

Notwithstanding the extremely minute dimensions 
of the chalk animalculae, their remains form rocks 
of great thickness in all parts of the world. In Europe 
we find, for example, extensive chalk formations in 
England, whose Latin name Albion was bestowed on 
account of the white chalk cliffs occupying long 
stretches of the coast. The hills of Champagne consist 
almost entirely of chalk; and Riigen, together with 
many other islands, is nearly all chalk cliffs. 



It is only in very rare cases, however, that chalk 
occurs in sufficient purity to be immediately suitable 
for use as a pigment or writing-material. For the most 
part it contains other minerals, or large fossils, from 
which it has to be separated by mechanical treatment. 
Nodular flints are often met with in chalk, and many 
deposits contain such large numbers of the petrified 
shells of the sea urchin that the chalk really cannot 
be used as a pigment at all, by reason of the high cost 

of purification. The only places where chalk can be 
advantageously worked for the preparation of pigment 
is where the mineral is in a high state of purity, and 
also contains only very few sandy particles. Such 
chalk deposits are worked on a mining scale, and, as 
a rule, in the state in which the chalk comes from the 
quarry ; it is in the form of a soft mass, easily scratched 
with the finger-nail and of fairly high density, owing 
to the considerable quantity of water with which it is 
ordinarily impregnated. 

In order to convert this crude chalk into a product 
that can be used as a pigmc nt, it is first left to dry 


until the lumps can be easily broken, and then crushed 
into small pieces, from which all the extraneous 
minerals, which occur as large lumps, are sorted out 
and removed. This picking process is important, 
especially when the chalk contains flints, because these 
latter are very hard and would injure the millstones 
in the subsequent grinding. 

The lumps of chalk are reduced by mechanical means, 
such as a stamp-mill, or, more frequently, in a mill of 
the same type as for grinding flour, since it is impossible 
to get the lumps so dry as to produce the degree of 
brittleness necessary for a thorough reduction in a 
stamp-mill. The millstones are enclosed in a wooden 
casing, and the chalk is ground in admixture with 
water, the ground mass escaping, through an opening 
in the casing, as a thick pulp which is stored for a 
considerable time in large tanks. 

Experience has shown that this method of prolonged 
storage in contact with water greatly improves the 
colour. The only explanation of this fact is that the 
chalk still contains a very small amount of organic 
matter, which gradually decomposes in presence of 
water. The evidence in favour of this is the peculiar 
smell given off during storage. 

Even with the most careful grinding, chalk cannot 
be transformed into such a fine powder that is directly 
fit for all purposes; and the only way to obtain the 
requisite fineness is by levigation. Owing to the large 
quantities that are usually handled in this process, the 
milky liquid coming from the mill is mostly run into 
large brick tanks, where it is left to settle until all the 
chalk has deposited and the supernatant water is 
perfectly clear. Tapping-off being usually imprac- 
ticable, the water is generally drawn off by careful 


syphoning, so as not to disturb the fine sludge at the 
bottom of the tank. 

The deposit in the settling-tanks is shovelled into 
wooden boxes, perforated at the sides to enable the 
water to drain away, the chalk being prevented from 
escaping by lining the boxes with linen cloths. The 
pulp soon loses its liquid character and shrinks con- 
siderably, the boxes being then filled up with more 
sludge, and so on until the contents have ceased to 
shrink. When the mass is so far dry that it will no 
longer run when lifted, the boxes are covered with 
boards and inverted, discharging the contents on to 
the boards, on which the mass is left to become quite 
dry. Filter-presses are also used. 

Large prismatic masses of chalk never dry so 
uniformly as to prevent the formation of cracks, and 
if the chalk is to be sold in this form the cracks are 
plastered up with thick pulp ; this operation, however, 
being superfluous when the chalk is to be sold as 

In order to obtain a more compact product and 
accelerate the drying of the moulded lumps, some 
makers use presses, in which the fairly dry chalk is 
subjected to progressive heavy pressure. 

Owing to the fineness of the component particles of 
chalk, they adhere so firmly together, without any 
bind, that a fair amount of force is necessary to break 
down a piece of perfectly dry levigated chalk. Some- 
times, however, chalk exhibits the unpleasant property 
of losing its cohesion almost completely when dry, and 
in such cases it can only be shaped into prisms with 
great trouble. This peculiarity is specially accentuated 
when the chalk contains magnesia; and in order to 
mould chalk of this kind into blocks, a binding agent, 


such as ordinary glue, must be added to the water used 
in grinding, care being taken not to use too much, or 
the chalk will become too hard, when dry, for certain 
purposes, e. g. as drawing or writing chalk. 

For some purposes, chalk is sold in powder form, 
and very high purity is not then essential, an admixture 
of magnesia or clay being harmless. Gilders, for 
instance, use large quantities of chalk for priming 
picture frames, and stir the chalk up with a certain 
amount of bind (mostly size), to give the particles the 
desired cohesion. 

The chief requirement exacted of a good quality 
chalk is a handsome white colour; and this depends 
entirely on the quality of the raw material, not on the 
method of preparation. It is known that a substance 
quite devoid of colour will furnish a perfectly white 
powder, because the colourless particles reflect the 
light in all directions without breaking it up into its 
constituent yellow, red and blue rays. Chalk, too, is 
in reality a colourless substance, and reflects light with 
greater uniformity in proportion as the fineness of the 
particles increases. Consequently, when one has a 
chalk that is not perfectly white, it can, nevertheless, 
be made to furnish a very handsome product by 
bestowing great care on grinding and levigation. 
Properly prepared chalk should be as fine as the finest 

When the colour of the best grades of chalk are 
compared with what may be termed pure white such 
as that of white lead, zinc white, permanent white 
a skilled eye will always detect a greyish or yellowish 
tinge in the former, even if obtained from the whitest 
Carrara marble. 

The grey tinge is due to the presence of organic 


matter, which cannot be eliminated by any known 
means, but which can be shown to exist by the fact 
that when such chalk is heated to incandescence in the 
air for a short time, the resulting burnt lime is pure 
white, the organic matter having been burned off. 
A yellow tinge is caused by minute traces of ferric 
oxide, which as also ferrous oxide almost invariably 
accompanies calcium carbonate ; and limestone free 
from determinable quantities of these oxides is of rare 
occurrence. Ferrous oxide does not reveal its presence 
in limestone unless in large proportion, its pale green 
colour being of low tinctorial power, whereas ferric 
oxide, which is a very strong colouring agent, can be 
more readily detected. 

To those who are engaged in the manufacture of 
white earth colours, however, it is quite immaterial 
whether a limestone or chalk contains ferrous oxide, 
because that oxide quickly changes into ferric oxide 
in the finely divided product, and a chalk which was 
originally pure white will become decidedly yellow in 
a short time. 

Fortunately, such a yellow-tinged product can be 
rendered perfectly white by simple means and at small 
cost, all that is necessary being to add a suitable 
quantity of a blue colouring matter. When this has 
been done, the chalk will seem pure white to even the 
most skilled eye. 

This result of adding a blue pigment is based on the 
well-known physical fact that certain kinds of coloured 
light produce white light when combined, the colours 
that give this effect being termed " complementary." 
A pure blue is complementary to a yellow with a reddish 
cast- e. g. ferric oxide and therefore a chalk that is 
tinged yellow by a small quantity of ferric oxide can 


be changed into a seemingly pure white substance by 
the addition of a blue pigment. 

The only pigments of use in this connection to the 
colour-maker are such as have very intensive colouring 
power and at the same time are low enough in price. 
Such substances are ultramarine, smalt and coal-tar 
dyes. Smalt is the best because its colour is unalter- 
able. In point of chemical composition, this substance 
is a very hard glass coloured blue by cobalt ous oxide. 
For improving the colour of chalk or any other white, 
the smalt must be in an extreme state of fine division, 
and levigated to an impalpable powder. Ultramarine 
can be used for the same purpose, but is not so 

To ascertain the correct proportion of blue pigment, 
it is advisable to make a systematic experiment, which 
is easily performed. Exactly 90 parts of the chalk 
in question are triturated with 10 parts of blue pigment 
in a mortar until the entire mass has become a perfectly 
uniform pale blue powder, which contains 10% of the 
blue ingredient. 

Several samples, each representing one hundred 
parts of the white pigment to be corrected are carefully 
weighed out, I part of the blue powder being added 
to the first sample, 2 parts to the second, 3 to the third, 
and so on, and the mixtures are compared with a 
standard white substance, such as best white lead or 
zinc white, to see which most nearly approaches the 
standar4 colour. It is then easy to calculate how 
much of the blue requires to be added to 100 or 1000 Ib. 
of the material to be corrected. 

The correction can be effected in several ways ; for 
instance, by grinding the blue pigment directly with 
the bulk, by adding it at the levigation stage, or mixing 


it with the dry, finished product. The first two methods 
are attended with certain drawbacks which render it 
difficult to obtain a perfectly uniform product, owing 
to the specific gravity of the blue pigments being higher 
than that of the whites. Consequently, when the two 
are mixed in presence of water as is always the case 
in grinding and levigation the heavier blue pigment 
settles down more quickly, and several strata can be 
clearly distinguished in the sediment. The upper 
layers will still have a decided yellow tinge the 
proportion of blue being too small for proper correc- 
tion- whilst the next in order will be pure white 
accurately corrected and those at the very bottom 
will be decidedly blue, because they contain the largest 
proportion of the blue substance. 

The most satisfactory results are obtained by dry 
mixing ; and this can be successfully practised when the 
colour-maker has a cheap source of power (such as 
water power) available. Where/however, costly power 
plant has to be provided, only the finest grades of 
white pigments can be improved in this way, the 
expense of labour being too high for cheap materials. 

As a pigment, chalk possesses many valuable 
properties. The organic structure of chalk gives it 
high covering power as a wash, a thin layer applied 
to a surface sufficing to mask the colour of the under- 
lying ground completely. The lime in chalk being 
combined with carbonic acid, its basic properties are 
so extensively weakened that chalk can be mixed with 
even the most delicate colours without fear of their 
shade being affected. A coating of pure chalk paint 
on any surface will never change colour in the air; 
and on this account, chalk is extensively used both as 
an indoor wash and by wall-paper manufacturers. 



Many chemical processes furnish soluble salts of 
lime that constitute a by-product of little value. These 
salts, however, can be advantageously utilised for the 
preparation of an artificial chalk which is preferable 
to the native article in many respects. For instance, 
where large quantities of calcium chloride solution 
are available, and soda can be purchased at a sufficiently 
cheap rate, they can be converted into artificial chalk, 
because these two substances react on each other, 
forming, on the one hand, calcium carbonate, which is 
precipitated as a very delicate, insoluble powder, and 
on the other, sodium chloride, or common salt, which 
remains in solution, according to the equation : 

CaCl 2 + Na 2 CO 3 = CaCO 3 + NaCl. 

If, however, these solutions were mixed together in 
a crude state, the resulting product would be of only 
low value as a pigment, being of a yellow tinge and 
never pure white. This is due to the fact that the 
impure lime salts, being waste products from chemical 
works, frequently contain fairly large amounts of 
ferric oxide, and the soda also is often so high in that 
impurity that the colour of the precipitated chalk is 
considerably impaired. 

Fortunately,, there is no difficulty in eliminating this 
ferric oxide by chemical means, and obtaining a product 
of superior colour to the best native chalk. This is 
effected by treating the perfectly neutral lime-salt 
solution with calcium carbonate, which causes the 
precipitation of the iron, a corresponding amount of 
lime passing into solution. 

In order to eliminate the ferric oxide from the lime- 


salt solution so completely that not even the most 
delicate chemical test known will be able to reveal 
any trace remaining, the solution is placed in a vat 
and stirred up with finely powdered chalk. If the 
solution contains any free acid, effervescence, due to 
the liberation of carbon dioxide, will take place ; and 
in such event the addition of chalk is continued until 
the free acid is all neutralised, and the added chalk 
sinks to the bottom undissolved. The chalk should 
be in slight excess, so that a decided sediment is visible 
at the bottom of the liquid when at rest. 

This deposit is stirred up again at intervals with the 
liquid for several days. When ferric oxide is present, 
the colour of the deposit will gradually change to a 
yellowish brown, through the precipitation of ferric 
hydroxide by the chalk; and in this way the final 
traces of iron can be removed. 

The liquid is then carefully drawn off, without 
disturbing the sediment, and the soda solution is run 
in so long as a precipitate of calcium carbonate con- 
tinues to form. The completion of the reaction can be 
ascertained by pouring a small quantity of the liquid 
into a tall, narrow glass, leaving it to clarify, adding a 
little more soda solution and observing whether any 
further precipitate is produced. On the other hand, 
it may be that an excess of soda has already been added 
in the precipitating tank; and this can be determined 
by testing a sample with turmeric paper blotting- 
paper soaked in a solution of the colouring-matter of 
turmeric root which is turned brown by alkaline 
reagents. Even in very dilute solution, soda will give 
this colour change, and the test is therefore very 
accurate. The complete precipitation of the lime in 
the solution can be ascertained by passing a small 


quantity through blotting-paper and treating it with 
a little acid potassium oxalate solution, which, if lime 
be present, will at once produce a strong crystalline 
precipitate of calcium oxalate, which is only very 
sparingly soluble in water. If the oxalate gives 
merely a slight turbidity, the residual amount of lime 
is so small that the process may be regarded as complete. 

Since carbonate of soda is usually much dearer 
than the lime-salt liquor, it is preferable to leave a 
small quantity of the lime unprecipitated. Given 
sufficient care in effecting the precipitation, and 
especially when fairly strong solutions are used, a 
brilliant white precipitate of calcium carbonate is 
obtained, which is in such a finely divided state that 
the minute constituent crystals can only be detected 
under a high magnifying power. 

This precipitated chalk being already in an extremely 
fine condition needs no further preparation, and, when 
washed, is ready for immediate use, forming a handsome 
pigment with excellent covering power. 

When precipitation is ended, the deposit is allowed 
to settle down, and the clear supernatant liquid is 
carefully drawn off so as not to disturb the delicate 
sediment, which is then stirred up thoroughly with 
clean water, left to subside, washed again, and then 
spread out to dry on cloths which are suspended by 
the four sides. The surplus water drains away and 
the residue gradually assumes the consistency of paste, 
in which condition it can easily be moulded to any 
desired shape. If left long enough to dry completely, 
it forms a very delicate powder, furnishing a pigment 
of excellent quality. 

If this precipitated chalk be moulded into prisms 
for sale, the blocks are laid on one of their broad sides 


until firm enough to turn over on to one of the narrow 
faces, slabs of gypsum being used as the supporting 
material, in order to ensure uniform drying. The 
gypsum absorbs water with avidity and thus dries the 
prisms evenly. 

A defect of these prisms is their great fragility ; but 
their strength may be improved by mixing a little 
very weak solution of dextrin to the mass after the 
last washing-water has been completely removed. In 
drying, the dextrin binds the material of the prisms 
sufficiently to keep them from breaking except under 
the influence of a fair degree of force. 


As already mentioned, calcium carbonate rarely occurs 
in a perfectly pure condition in Nature; and chalk, 
also, is frequently contaminated by other minerals. 
A variety of limestone occurring as extensive deposits 
in many places is that in which calcium carbonate 
is associated with clay. Sometimes the clay pre- 
dominates, and the mineral is then known as marl, 
being really a clay contaminated with chalk. If, 
on the other hand, the chalk forms the chief constituent, 
the mineral is termed calcareous marl. 

Calcareous marls are used in much the same way as 
limestone, some modification, however, being necessi- 
tated by the presence of the clay. Although limestone 
containing a certain amount of clay can be burned in 
the kiln, it yields an inferior lime that is of little use to 
the builder owing to its low binding power. Marl 
of a certain composition finds an important application 
in the manufacture of hydraulic lime or cement. 

The only kind of marl suitable for pigment is that 


containing clay with very little colour; and this is of 
somewhat rare occurrence, because most marls contain 
sufficient ferric oxide to give them a yellow shade. 
Marl that is fairly free from ferric oxide, however, can 
very well be used as pigment ; and many white pigments 
sold as " chalk " are really finely ground marl. 

In accordance with the general practice, in the colour 
industry, of giving colours a great variety of names, 
and suppressing the real names, which, so far as the 
artificially prepared colours are concerned, should 
bear some reference to their chemical composition, 
numerous white earth colours bear fancy names, though 
really consisting of chalk, lime (generally marl), or 
white clay. 

In France, where both chalk and clay are of frequent 
occurrence the soil of Champagne, for instance, 
being all chalky the manufacture of the white earth 
colours is extensively practised, and a large number are 
put on the market, usually named after the place of 
origin, and consisting of either calcium carbonate or 

The trade names of the white earth colours include 
Cologne chalk, Bologna chalk, Briancon chalk, 
Champagne chalk, Blanc de Bougival, Blanc de Meudon, 
Spanish white, Blanc d' Orleans, Blanc de Troyes, etc. 
All are either more or less pure chalk, marl, or a fairly 
white clay, pipeclay which is also used for making 
clay pipes and for removing grease spots. 


The mineral known as gypsum, or alabaster, consists 
of calcium sulphate, or sulphate of lime, its composition 
being expressed by CaSO 4 + 2H./X In gypsum the 


crystalline structure is just discernible, whilst other 
varieties, such as the so-called " marine glass," occur 
in considerable quantities as large, perfectly transparent 
masses. " Russian glass " consists of large, trans- 
parent lumps possessing the specific property of 
gypsum, viz. that of cleaving in two directions, in a 
high degree. Alabaster is composed of finely granular 
masses, which are either quite white, or else yellowish, 
or traversed by grey veins. This variety of gypsum is 
very abundant in central Italy, and the best blocks 
are employed for the production of works of art. 

Ordinary gypsum, which frequently occurs in the 
vicinity of dolomitic limestones, is found in a great 
variety of colours, bluish-grey, yellowish or reddish 
tints being the most common. Pure white lumps, 
which are plentiful in some deposits, can be used as 
white pigment, the method of preparation being simple, 
viz. merely reducing the mass to powder. This is 
easily effected, the specific hardness of gypsum being 
only 2 ; and in many cases it is soft enough to scratch 
with the finger-nail. 

If the original gypsum is white, the powder forms a 
dazzling white flour which, notwithstanding, is of 
comparatively little value as a pigment, on account of 
its low covering power. For this reason, powdered 
gypsum is chiefly used for making plaster of Paris 
(calcined gypsum) for plaster casts and stucco. Gypsum 
may also be employed to advantage for lightening 
various colours, since it is inert towards even the most 


Large areas of the earth's surface are covered with 
clay, which often attains a considerable thickness. 


Nevertheless, the kind of clay that is suitable for use 
as pigment is comparatively scarce. The principal 
requirement for this purpose is a pure white colour, 
but by far the great majority of clays are either yellow 
or of a shade between blue and grey (for example the 
clay of the Vienna basin). 

The character of clay is just as varied as its colour. 
In some places, large deposits of extremely fine clay 
are found, the material, when mixed with water, 
forming a highly plastic mass which, when dried and 
subjected to slight pressure, furnishes a very soft 
powder. On the other hand, some clays are so inter- 
spersed with large quantities of sand, large stones and 
the debris of mussels, that they cannot be used until 
they have been put through very careful mechanical 

This great divergence in the physical character of 
clays is due to their method of formation. Clay 
originated in the weathering of felspar, which chiefly 
consists of a double salt, a compound of the silicates 
of alumina and potash. Under the influence of air 
and water, this compound is decomposed, the potassium 
silicate passing into solution, whilst the aluminium 
silicate, being insoluble in water, is carried away by 
that medium. When the water can no longer carry 
the particles of aluminium silicate in suspension for 
example when it reaches a sea or lake the silicate 
settles down to the bottom, and a deposit of clay is 

If the original felspar was very pure, and in particular 
very low in iron, the resulting clay will be of a handsome 
white colour. An example of this is afforded by 
kaolin, or porcelain earth, which is preferably used for 
making china. If, however, the felspar contained a 


considerable proportion of ferric oxide, the resulting 
clay is yellow; and if stones or mussel shells became 
incorporated with the clay prior to deposition, these 
bodies will be found as inclusions in the deposit, and 
such clay will require much troublesome preparation- 
grinding and levigation before it is fit for use. 

For the purposes of the colour-maker, the most 
suitable clay is one that is pure white, free from inclu- 
sions, and does not change colour when exposed, in a 
finely divided state, to the action of the air. Many 
clays that were originally white gradually assume a 
yellow tinge on prolonged exposure to air and moisture, 
because the clay contained ferrous oxide, which 
changes, in the air, to the stronger pigment, ferric 

Many kinds of clay merely require a simple levigation 
to fit them for use as pigment. The lumps of freshly 
dug clay are placed in large tanks, etc., filled with water 
and stirred up continuously in order that, instead of 
forming a plastic mass which is very difficult to dis- 
tribute in water, the particles detached from the lumps 
may pass at once into suspension. This turbid water 
is then transferred to another tank, etc., where the 
minute particles of clay are allowed to settle down, and 
the water becomes quite clear. 

Where this work is carried on on a large scale, it is 
advisable to put the freshly won clay into large pits 
close to the clay deposit, and to leave it there, covered 
with water, during the winter season. The freezing 
of the water breaks down the larger lumps of clay, 
by the resulting expansion, and this facilitates the 
subsequent levigation, the cohesion between the 
particles being destroyed. 

If the clay contains larger proportions of lime or 


magnesia, a little experience will enable their presence 
to be detected at once by the way the clay behaves 
on being placed in contact with water. Pure clay 
quickly forms a fatty and extremely plastic paste, and 
sticks closely to the tongue when applied in the dry 
state. On the other hand, clay containing much lime 
or magnesia is far less plastic when mixed with water, 
and the dry clay hardly adheres to the tongue at all. 

These latter clays are classed as poor or lean, in 
contrast to the fat, plastic kinds. For certain purposes 
for which clay is used as pigment, these admixtures 
are not harmful ; whereas others, especially quartz 
sand and mica, not infrequently present in white clays, 
constitute a serious drawback. 

As already mentioned, clay is formed by the weather- 
ing of felspar, which is a constituent of granite and 
gneiss, both rocks composed of quartz, mica and felspar. 
When the clay has been derived from the weathering 
of such rocks, it is easy to understand that it may 
contain admixtures of quartz and mica, which are 
frequently visible to the naked eye, or at any rate 
under the microscope. Whereas clay forms a white, 
amorphous mass, the grains of quartz sand are decidedly 
crystalline, transparent and of vitreous lustre; the 
scales of mica, on the other hand, appearing as thin 
tabular crystals, mostly of a green or brown colour 
and exhibiting, when viewed at certain angles, a 
brilliant metallic sheen. 

Quartz sand can be eliminated from clay witnout 
any special difficulty, quartz being of higher specific 
gravity and therefore settling down quickly, leaving 
the delicate particles of clay in suspension in the liquid. 
The scales of mica are harder to get rid of, their tabular 
form retarding deposition from the suspending liquid ; 


and on this account, several washings are often required 
to separate them completely. 

In all cases where clay is to be used as a white 
distemper, the presence or absence of lime is immaterial ; 
but where it is to be employed for removing grease, 
lime is a drawback. This is also sometimes the case 
when the clay is wanted for the purposes of the colour 
manufacturer. The author has found, by experience, 
that perfectly pure, white clay forms a good paint, 
in a vehicle of oil or varnish a purpose to which it 
has, so far, been seldom applied, if at all. Such paint 
is of good covering power, and possesses the valuable 
property of remaining quite unaffected by atmospheric 

If, however, the clay contains even but a small 
quantity of lime, it cannot possibly be used as an oil 
or varnish paint, for though the freshly made paint 
has a very good appearance, its character soon changes, 
turning viscous and suffering a considerable diminution 
of covering power. Thinning with turps or boiled 
oil results in the formation of small lumps, so that it 
is quite impossible to obtain a uniform coating on even 
a small surface. 

This behaviour is apparently due to the presence of 
the lime, the explanation being that the fatty acids 
always present in the oils and varnishes used for the 
paint combine with the lime to form compounds 
which, from the standpoint of the chemist, must be 
regarded as soaps. The small lumps already mentioned 
really consist of lime soap, and the formation of these 
colourless compounds accounts for the lessened 
covering power. 

Given a fine white clay, otherwise capable of forming 
a valuable pigment, it is sometimes possible, by simple 


means, to eliminate accompanying lime, provided 
the amount of the latter is not too great, and also 
provided that very cheap hydrochloric or acetic acid 
is available. The acid need not be pure, and the impure 
but very strong pyroligneous acid, which is very 
cheap on account of its empyreumatic smell, may be 

To eliminate lime from the clay, the still moist 
levigated mass is introduced, in small quantities, into 
a vat containing the requisite quantity (see later) of 
hydrochloric or acetic acid, the addition being con- 
tinued until the liquid gives only a faintly acid reaction 
with blue litmus paper. When the clay is run in, 
effervescence is produced by the liberation of the 
carbon dioxide displaced by the stronger acid employed. 

The amount of lime present in a clay may be deter- 
mined by very simple means. A small sample of the 
clay is dried by artificial heat, until of constant weight, 
and exactly 100 parts by weight of the dry mass are 
placed in a glass and suffused with hydrochloric acid, 
sufficient of the latter being used to make the liquid 
still strongly acid after effervescence has ceased. 

The contents of the glass are transferred to a 
filter, and washed with pure water so long as the 
washings continue to redden blue litmus paper. 
The residue is then dried until of constant weight, and 
the difference between the initial and final weights 
will give the percentage of substances soluble in 
hydrochloric acid. 

After performing this simple test on a clay, it is 
easy to calculate the quantity of acid needed to extract 
all the soluble constituents from a given weight of 
the material. All that is necessary is to measure 
the volume of acid required to extract a small quantity 


of the clay completely. Thus, if one pint of the acid 
at disposal is sufficient to treat one pound of the clay, 
the amount needed for a given quantity of clay is a 
simple matter of calculation. 

Since, on account of the cost of pure hydrochloric 
acid, crude acid will always be used, it will be necessary 
to remember that this crude acid always contains 
ferric oxide in solution this being the cause of its 
yellow colour. If the amount of acid taken is barely 
sufficient to combine the whole of the lime, leaving 
the latter slightly in excess, the ferric oxide which 
would otherwise tinge the clay yellow will be 

If, on the other hand, the acid is in excess, the 
clay is obtained free from all constituents soluble in 
the acid. The purified clay must then be freed from 
the calcium chloride, formed by dissolving the lime, by 
a thorough washing, since the clay would otherwise 
always remain moist on account of the hygroscopic 
properties of the chloride in question. Moreover, any 
small residuum of free acid would constitute a draw- 
back on the clay being mixed with other colours. 

Calcium chloride is very soluble in water, and there- 
fore can be completely removed from the clay by 
washing. The purified clay is left to settle down as 
completely as possible, and after drawing the liquid off 
from the sediment, the latter is suffused with pure 
water and left to settle once more. As a rule, two such 
washings will cleanse the clay of calcium chloride and 
free acid sufficiently to render the product suitable 
for any purpose. 

When large quantities of clay have to be treated 
in this manner, considerable amounts of calcium 
chloride solution will be obtained, which can be advan- 


tageously utilised for the production of precipitated 
chalk, all that is necessary being to collect the liquor 
in a large tank and treat it with a small quantity of 
slaked lime, to transform the surplus free acid into 
calcium chloride and precipitate the ferric oxide present 
in solution. At the end of a few days the liquor in 
the tank will consist of a very pure solution of calcium 
chloride which will furnish an excellent precipitated 
chalk when treated in the manner already described 
under that heading. 


This mineral chemically, barium sulphate, BaSO 4 
occurs native, as extensive deposits, in many places 
England, Bohemia, Saxony, Styria, etc. It sometimes 
forms handsome tabular crystals, but more frequently 
compact masses, which may be pure white, grey yellow, 
etc., in colour, and are distinguished by high specific 
gravity (usually 4-3-4-7), to which the mineral owes its 
name. This high density also limits the application 
of the mineral, and it cannot be used as a pigment, 
in the true sense of the term, being only suitable as an 
adjunct to artificially prepared colours. 

The employment of barytes in the colour industry 
is often regarded as adulteration, which, however, it 
is not when the case is considered from the right point 
of view. For instance, the only preparation which can 
properly be termed white lead consists of basic lead 
carbonate. This, when pure, is a rather expensive 
pigment, whereas, for certain purposes, the consumer 
requires a product that can be obtained at a low price. 
In order to satisfy this demand, the only course open 
to the colour-maker is to mix the white lead with a 


cheap white substance, which enables him to turn out 
different grades of white lead, which, although low in 
price, are far inferior to the pure article in covering 
power. Pure white lead being itself a very heavy 
substance, the only bodies suitable as adjuncts are 
such as are also of high specific gravity; and of all 
the cheap pigments known, heavy spar is the only 
one endowed with this property. Consequently, this 
substance is extensively used in making the cheaper 
grades of white lead and the pale kinds of chrome 

The only cases in which the addition of heavy spar 
to a colour can be regarded as an intentional fraud on 
the consumer is when he is sold, as pure white lead, 
chrome yellow, etc., a product really composed of a 
mixture of such colour and barytes. Moreover, the 
presence of barytes in white lead can be easily detected 
by a simple examination, pure white lead readily 
dissolving, with considerable effervescence, in strong 
nitric or acetic acid, whereas barytes is insoluble in 
all acids, and therefore remains, as a heavy white 
powder, at the bottom of the vessel. In this way 
both the presence and amount of barytes contained 
in a sample of white lead or chrome yellow can easily 
be ascertained. 

The preparation of barytes for the purposes of the 
colour-maker is entirely a mechanical operation. The 
barytes, which though fairly hard is easily reduced, 
is crushed with stamps, ground in a mill and finally 
levigated, it being impossible to obtain a sufficiently 
fine powder even by repeated grinding. 

Native barytes must not be confounded with the 
artificial barium sulphate sold as permanent white 
or blanc fixe, which is an extremely finely divided 


barium sulphate obtained by precipitating a solution of 
a barium salt with sulphuric acid or a soluble sulphate, 
and is a painters' colour that is highly prized for certain 
purposes. Both the native sulphate and the artificial 
variety have the property of remaining completely 
unaltered by exposure to air, and they can therefore 
be mixed with any kind of pigment without fear of 
the colour deteriorating. 

As a rule, barytes is first roughly crushed in edge- 
runner mills or stamps, and then ground to the extreme 
degree of fineness obtainable in ordinary mills. Even 
with the greatest care, however, it is impossible by this 
means to obtain sufficient fineness of division for mixing 
with fine colours, the only way in which this can be 
accomplished being by levigation. 

Given a fairly pure white barytes to begin with, 
levigation furnishes a handsome white pigment that 
can be mixed with colours of any kind; but when 
used by itself in association with oil or varnish, its . 
covering power is very low and the colour never 
perfectly white. Native barytes is therefore unsuitable, 
as such, for paint. 

Varieties that are not pure white are sometimes 
corrected with ultramarine, added in the grinding- 
mill. If the yellow tinge is due to iron compounds, 
this can often be remedied by treating the finely ground 
material with hydrochloric acid, which dissolves 
them out, this treatment being followed by a thorough 
washing with pure water. 

As already mentioned, white lead is most frequently 
mixed with barytes, this being usually added when 
the white lead is being ground, by feeding the two 
materials to the mill and grinding them together. 

The crudeness of mechanical methods of reduction 


is clearly exemplified by comparing the most carefully 
ground and levigated barytes with that obtained 
by artificial means. The permanent white largely 
used in the production of wall-paper, and quite unalter- 
able in air, is, chemically speaking, identical with 
native barytes, viz. barium sulphate. The two also 
seem to be identical in crystalline habit, as is usual 
in the case of one and the same mineral, whether native 
or prepared by artificial means. Artificial barytes 
is obtained by treating a soluble salt of barium with 
sulphuric acid, or a solution of sodium sulphate (Glauber 
salt), so long as a precipitate continues to form. 

This precipitate is barium sulphate, which subsides 
completely on account of its extreme insolubility, this 
being greater than that of any other salt known. The 
rapid rate of deposition results in the formation of 
extremely small crystals, which, being colourless and 
reflecting the light completely, appear to be perfectly 
white. Even when permanent white is applied in 
very thin layers to any surface, its covering power is 
very considerable, by reason of the extremely fine sub- 
division of the material. 

This behaviour of artificial barytes in comparison 
with that of the natural product, affords an important 
hint in connection with the preparation of earth colours, 
namely, that in order to obtain products of specially 
good quality, the endeavour should be to reduce the 
raw materials to the finest condition possible. This 
result is accomplished most securely by bestowing 
the greatest care on grinding and levigation ; and it 
is therefore highly important that the manufacturer 
should select, from the various apparatus used in 
reducing the materials, those that are best adapted 
for the purpose. 



Although carbonate of magnesia is seldom used 
alone as a pigment, it can be advantageously employed 
as such when circumstances permit. It is met with 
not infrequently, in Nature, in a crystalline form, as 
magnesite or bitter spar, the latter name arising from 
the fact that the soluble salts of magnesia have a bitter 
taste. Still more frequently, magnesia occurs in 
association with calcium carbonate, in the mineral 
dolomite, which contains up to 20% of magnesia. 

A less abundant native mineral is hydromagnesite, 
which consists of basic magnesium hydrocarbonate. 
Hydromagnesite is a very light, chalk- white mass, 
with a non-greasy feel, which, when reduced to a 
soft powder, forms an excellent material for paint. 
It is highly inert, in a chemical sense, and can therefore 
be mixed with the most delicate colours, having no 
other effect thereon than to render them lighter in 

This product can also be prepared artificially, by 
treating a dissolved magnesium salt with a solution 
of carbonate of soda, the result being the formation 
of a pure white precipitate, which is very brilliant 
when dry, and is characterised by unusually low specific 
gravity. In some places, conditions are such that 
this preparation can be made on a large scale at very 
low cost. For instance, there is a spring at Bilin, in 
Bohemia, the water of which contains large quantities 
of alkali carbonates in solution ; whilst in the vicinity 
of Saidschlitz is a spring fairly rich in magnesia salts. 
The waters from these two springs are concentrated 
by evaporation, and mixed in large tanks ; and when a 
sufficient deposit of the resulting basic carbonate of 


magnesia has accumulated, it is taken out of the tanks, 
placed on linen niters and washed with water. The 
residue is dried slowly, without the employment of a 
high temperature, and then forms a white powder, 
which is very light and can be used for a number of 
purposes, chiefly medicinal, though it is also well 
adapted as a material for paint. 

For this latter purpose it is, however, far too expen- 
sive; but since the conditions obtaining at Bilin are 
certain to occur elsewhere, we have included carbonate 
of magnesia among the earth colours. 

On account of its specific lightness, carbonate of 
magnesia is specially adapted for making pale shades 
of certain delicate lake colours, which, if toned with 
even perfectly pure chalk, would undergo alteration in 
course of time. Carmine, for instance, can be graded, 
by the addition of carbonate of magnesia, into every 
possible variety of shades between the pure red of 
carmine itself and the palest pink; and the resulting 
colours are quite permanent whether mixed with gum 
solution or any other vehicle. 


Although this mineral is not used as a pigment by 
itself, it must be mentioned here because it is not 
infrequently employed for mixing with other colours, 
and is also used in the wall-paper industry. It also 
serves to distribute certain pigments in a state of fine 
division, the " rouge vegetal " of the perfumer, for 
example, usually consisting of talc and a small quantity 
of very fine carmine. 

In commerce the name talc is sometimes applied 
to two separate minerals, true talc and steatite or soap- 


stone. The former is rarely met with native as well- 
defined crystals, mostly occurring as scaly masses in 
primitive rocks. Thin pieces exhibit a certain degree 
of flexibility. The hardness of this mineral is so small 
that it can be scratched with the finger-nail; and its 
sp. gr. is 2'9-2-8. Talc is easily scraped, and the 
powder remains sticking to the knife, a property which 
renders the substance difficult to reduce to powder, 
because it balls together and takes a very long time to 
convert into a fine flour. The process is facilitated by 
calcining the talc and quenching it in cold water, this 
treatment increasing the hardness and at the same time 
making it more brittle, and thus more easy to pulverise. 
A characteristic feature of all the talc minerals is 
their peculiar greasy appearance and feel. The colour 
varies, white pieces alone being of any use to the colour 
manufacturer. The yellow- or green-tinged varieties 
owe their shade to the presence of ferric and ferrous 
oxides. In chemical composition, talc consists of a 
combination of magnesium sillicate with hydrated 
silica, the supposed formula being : 4MgO . SiO 2 + 
H 2 O . SiO 2 , and the percentage composition : silica, 
62-6% ; magnesia, 32-9% ; water, 4-9%. 


Steatite so closely resembles talc in most of its 
properties, that the two minerals were long regarded 
as identical. Whereas, however, talc is scarcely acted 
upon at all by the strongest acids, steatite is completely 
decomposed by prolonged boiling therewith, although 
both minerals have exactly the same composition. 

As a pigment, steatite is far more important than 
talc, and, as French chalk, is largely used for drawing 


or writing. To prepare it for this purpose pure white 
steatite requires no preliminary treatment, beyond 
cutting the large lumps up into quadrangular prisms, 
which are mounted in wood, like lead pencil, and used 
for writing on the blackboard. The powder produced 
in the cutting process is made up into pastel crayons. 
With this object, the powder is mixed with a sufficient 
quantity of some mineral pigment to produce a mass 
of the desired shade, and is kneaded to a stiff paste with 
water containing an adhesive such as gum, glue or 
tragacanth mucilage. The mass is shaped into prisms, 
which, when dry, are cut into pencils and mounted 
in wood. Steatite being like talc, without action on 
even the most delicate colours, can be used as a diluent 
in the preparation of light shades. 



ALL the yellow earth colours, without exception, 
have ferric oxide as their colouring principle, the 
differences in shade being entirely due to the varying 
proportion in which that oxide is present. The various 
names under which they are known date back to a 
period when the chemical nature of these colours was 
still unknown, and have been mostly derived from the 
locality of origin. 

'file yellow earths can therefore be divided into two 
groups, according to their chemical character. The 
first group, in which the ferric oxide is present as 
hydroxide, comprises all the ochres, Siena earth, and 
a number of others which are obtained from native 
ochre by special treatment. In the colours of the 
second group, ferric oxide is still the colouring principle, 
but is combined with other substances in place of water. 

It is, as a matter of fact, incorrect to rank the ochres 
in general as yellow earths, because they can be made 
to yield nearly every variety of colour from the palest 
yellow to the deepest red, brown and violet. These 
colours merit the particular attention of the colour- 
maker and the painter, being distinguished by very 
low cost of production, unusual permanence and beauty 
of tone. In the interests of that highly important 
matter to the artist, namely the production of colours 



of unlimited permanence, it is desirable that colour 
manufacturers should bestow greater care on the 
manufacture of these colours than has hitherto been 
the case. An extremely favourable point about nearly 
all these pigments is that they can be very cheaply 
prepared by artificial means, so that the manufacturer 
is in a position to turn out a large number of the hand- 
somest and most durable colours with a small amount 
of expense and labour. 


Ochres are found in many localities, most frequently 
in stratified rock and rubble. The deposits are rarely 
extensive, mostly occurring in pockets or beds. Where- 
ever found, ochre may be termed a secondary product, 
that is to say, one that has been formed through the 
destruction of other minerals. The analysis of ochres 
from different deposits shows great divergence in 
composition ; and some consist almost entirely of pure 
ferric hydroxide, that has already undergone natural 
levigation and can be used as a pigment as soon as dug. 

Such a form is, however, rare, and most ochres are 
intermixed with smaller or larger amounts of extraneous 
minerals, the contamination being sometimes so great 
as to preclude the use of the ochre as pigment by reason 
of the high outlay required for extracting the colouring 

Occasionally, the ferric hydroxide is associated with 
a certain proportion of clay, and as this increases, the 
ochre passes over into ferruginous clay. This class 
can also be used as pigment, in certain circumstances, 
that is to say when it is sufficiently rich in ferric oxide 
to furnish a deep red mass on calcination. When, 



however, the proportion of ferric oxide is low, its 
pigmentary power is no longer sufficient, and the clay 
has not the requisite beauty of colour. The ordinary 
earth used for making tiles is an example of this class, 
its colour in the raw state being an ugly brownish- 
yellow, but turning a dull " brick " red when fired. 

In some deposits the ferric oxide is accompanied by 
lime. Unless the latter exceeds a certain proportion, 
such ochres, too, are suitable as pigments, the lime 
being easily removed by simple levigation ; but when 
the amount of lime is high, it is difficult to obtain 
certain highly coloured shades of ochre from such 
material. These shades entail the calcination of the 
ochre, and the temperature required is oftentimes 
insufficient to transform the lime into the caustic state. 
Moreover, the presence of caustic lime would be a 
drawback in some cases, it being then impossible to 
mix the ochre with other colours without endangering 
the shade through the action of the lime on these latter. 

The following analyses will show the percentage 
composition of ochres from various deposits : 

Ochre from 



St. Georges. 

Ferric oxide 




Lime .... 


Alumina .... 


Magnesia .... 
Silica .... 




Water . . 




In the majority of cases the mineralogical character- 
istics of an ochre enable conclusions to be formed as 
to its suitability as pigment. Good ochre is more or 


less yellow to dark brown in colour, and can easily be 
crushed between the fingers to a soft, fine powder 
which feels like powdered steatite and does not produce 
a sensation of grittiness, this latter indicating the 
presence of fine grains of sand in the ferric oxide. The 
behaviour of the ochre in presence of water is specially 
important. If it adheres firmly to the tongue, and 
forms a fairly plastic paste when mixed with a little 
water, the mineral contains a large percentage of ferric 
oxide, and as a rule will yield ochre of good colour. 

In general it may be said that the value of an ochre 
varies directly with its content of ferric hydroxide or 
oxide, because when this is large the ochre will furnish 
a wide range of colours under suitable treatment. 

A simple test for quality consists in weighing out 
an exact small quantity (10 grms.), and heating it to 
a temperature not exceeding 110 C., until the weight 
remains constant. A simple calculation then gives the 
amount of uncombined water in the sample. Since 
the proportion of such water varies in different parts 
of one and the same deposit, the test must be repeated, 
in order to obtain accurate results, on samples taken 
from different points, or, preferably, on a properly 
prepared average sample. 

Even drying changes the colour of ochre considerably. 
To ascertain the behaviour of an ochre on calcination, 
a large sample is dried at 110 C. until the weight is 
constant, and divided up into a number of small samples 
weighing, say, 10 grms. each. The samples are then 
heated to different temperatures, one to the melting- 
point of lead, another to that of zinc, and so on. 

The higher the temperature employed, the more will 
the colour of the ochre approximate to red ; and 
specimens very rich in ferric oxide will give bright red 


colours. Beyond this range, a further increase in 
temperature will give violet shades, varying with 
the temperature and the duration of heating. After 
this preliminary test, it is desirable to make another 
on a larger scale, with quantities up to about i Ib. 
For this test, the different kinds of ochre frequently 
found in the same deposit should be mixed together, 
in order to obtain an idea of what the mean product, 
obtained in working on the large scale, will be like. 

On the whole, the results of this second test will be 
the same as in the first series, the only object of the 
second test being to gain information which may be 
particularly valuable in practical work. The bottles 
in which the calcined samples are stored should be 
marked with the temperature and length of heating, so 
that, when it is subsequently desired to obtain an ochre 
corresponding to a particular sample, all that is 
necessary will be to heat it to the same degree from 
the same length of time. The performance of this 
simple test will be of great assistance in standardising 
the work with a minimum loss of time. 

When it is desired to ascertain the composition of 
an ochre superficially its behaviour towards hydro- 
chloric acid maybe noted. A weighed quantity of the 
freshly dug (undried) ochre is treated with pure acid, 
free from iron, which will dissolve out the ferric oxide 
and lime, leaving clay and quartz sand behind. The 
presence of lime is indicated by effervescence on contact 
with the acid ; and if there is no effervescence, lime is 
absent. At the end of several hours the acid is care- 
fully decanted from the undissolved residue which is 
then stirred up with water, left to subside, and weighed 
when dry. This method will give the amount of 
substances, other than ferric oxide and lime, in the 


sample. These substances usually consist of clay or 

For a quantitative determination, a small quantity 
usually I grm. is weighed out, treated with a 
corresponding amount of hydrochloric acid, and the 
solution filtered into a glass. The residue on the filter 
is washed with distilled water, the washings being 
united to the acid solution. 

This solution is treated with ammonia so long as a 
precipitate of ferric hydroxide continues to form, this 
being collected on a tared filter and dried at 110 C. 
The precipitate may be regarded as pure ferric 
hydroxide, and its weight will indicate the proportion 
of hydroxide in the ochre with sufficient accuracy 
for technical purposes. 

In reality, however, it is not pure ferric hydroxide, 
but contains in addition all the oxides that are precipi- 
table by ammonia, lime being always carried down as 
well. It is therefore desirable to dissolve the precipitate 
with a little hydrochloric acid, and reprecipitate with 


In many places ochre is only put through a very 
simple mechanical preparation before being sold for 
pigment, namely left to dry in the air so that most of 
the uncombined water evaporates. No matter how 
this drying process is protracted, however, it is impos- 
sible to get rid of all the water in this way, a certain 
proportion being retained by the hygroscopic action 
of the ferric hydroxide, and to expel this the mass 
must be heated to above 100 C. Drying is usually 
succeeded by pulverising and sifting the loose earthy 
mass, which is then ready for sale. 


When the ochre contains sand or stones, this treat- 
ment is not sufficient, and levigation is necessary. No 
particular trouble is involved, the mineral being fairly 
heavy as the result of its content of ferric hydroxide. 
A simple method of treatment suffices to improve the 
value of the ochre considerably, and enables a grade 
that is not particularly bright-coloured in its natural 
condition to be converted into products of very hand- 
some tone and various shades. This treatment con- 
sists in heating the raw ochre to a definite temperature, 
during which process the colour changes progressively, 
and any desired tone can be obtained by suddenly 
cooling the hot mass. 

The reason for this phenomenon is that the higher 
the temperature, the larger the amount of water driven 
off from the ferric hydroxide, until finally, when a very 
high temperature has been reached, the whole of the 
water is expelled, and the ferric hydroxide is trans- 
formed into ferric oxide. The hydroxide is brown, 
whereas the oxide, provided the temperature has not 
been raised too high, exhibits the characteristic colour 
known as " iron red." 

Consequently, the colour of moderately calcined 
ochre ranges through a whole scale frcm brown to red ; 
and the higher the temperature employed, the redder 
the tone. If the heating be protracted after all the 
hydroxide has become oxide, the latter undergoes 
molecular change, increasing considerably in density 
and altering in colour; and after very prolonged 
heating, the colour finally becomes violet. 

The calcination, or burning, of ochre is ordinarily 
performed in a very crude manner. The mineral is 
crushed to the size of peas, and spread out on an iron 
plate which is made red-hot. As soon as the ochre 


has reached the desired shade of colour, it is dropped 
into a tub of water and then crushed to powder. The 
calcination requires great experience on the part of 
the operator, because so long as the product is hot, it 
has quite a different colour from that assumed on 
complete cooling. Since only comparatively small 
quantities of ochre can be treated in this way, and the 
operation unnecessarily increases the cost of the 
product, owing to the large consumption of fuel, it is 
highly desirable to employ a simple calcining apparatus 
capable of treating large quantities. 

Such an apparatus may consist of an iron drum, 
mounted with a gentle slope inside a furnace, from 
which it projects at both ends. A shaft carrying a 
sheet metal worm is rotated inside the drum; and the 
whole apparatus is very similar to an Archimedean 

When the iron drum is raised to a strong red heat, 
and small quantities of ochre are fed continuously into 
the upper end of the drum, the rotation of the worm 
will push the material forward, and contact with the 
glowing sides of the drum will produce the necessary 
calcination, the degree of which can be modified by 
altering the speed at which the worm is turned. The 
calcined product is discharged at the lower end of the 
drum, either into a vessel of water, or, if only moderate 
heating has been applied, direct into a collector. 

Fig. 28 represents an apparatus designed by Halliday 
for the dry distillation of wood waste ; but, with slight 
structural modifications, it can also be used for calcining 
ochre. The material to be heated is introduced, in 
small pieces, into the feed hopper B, and is carried 
downward, by the worm C, into the red-hot drum A, 
through which it is propelled by the worm D until it 



drops out, at F, into the tank G. The length of time 
the material is subjected to calcination depends on the 
speed at which the worm D is run. The pipe E carries 
off the water vapour expelled from the charge. 

In order to obtain a uniform product when ochre is 
calcined in an apparatus constructed on this principle, 
it is necessary that the material introduced should be 

FIG. 28. 

fairly regular in size, a condition which is easily fulfilled 
by squeezing the freshly dug ochre between fluted 
rollers, and then passing it over a series of screens, each 
grade being then calcined separately. 

Moreover, the apparatus is only suitable for calcining 
at medium temperatures; and when highly calcined 
products are in question, the operation is best performed 
in fire-clay cylinders, or in thick cast-iron drums, similar 
to gas retorts, built into a furnace. 


Other devices for calcining ochre will be described 


As previously stated, ochres are frequently met with 
in! Nature, both in the immediate vicinity of iron ore, 
and also at considerable distances from such deposits. 
In the latter case, the ochre must be assumed to be 
the decomposition products of ferruginous minerals 
and to have been carried off by water until the latter 
became stagnant and allowed the ochre to settle down. 
In their method of deposition these ochres are therefore 
analogous to clay, and they, too, often contain large 
quantities of extraneous minerals, which have given 
rise to the diversified substances grouped under the 
name of ochre. 

Although ochres are so widespread in Nature, only 
certain kinds, found in certain localities, have acquired 
a high reputation. For the most part, these ochres 
are such as have already been prepared in a high 
degree, by Nature, for the purpose for which they are 

Thus, we find that all the ochres which have acquired 
a high repute among painters for particular beauty 
of tone and permanence, are distinguished by two 
properties : a high content of ferric hydroxide and great 

The former of these properties imparts brightness 
of colour; and such products will furnish, on calcina- 
tion, a wide range of colour shades. When, as is the 
case with the finer qualities of ochre, the mineral 
contains only a very small proportion of impurities, 
there is no difficulty in bringing it, by simple grinding 


or levigation, into a condition in which it is at once 
fit for use as a pigment. 

The Italian ochres have, for long ages, enjoyed a 
high reputation for their beauty of colour and per- 
manence. This category includes, for example, the 
renowned Siena earth, Roman earth, Italian umber, 
and other ochre colours. This high renown is probably 
due less to the inherent properties of the mineral than 
to the circumstance that the art of painting attained 
a high state of development at an early period, and 
that the artists paid special attention to the use of 
bright and permanent colours for their work. 

Although, at present, many deposits of ochre are 
known that are quite able to compete, on the score of 
beauty, with the best Italian products, the good name 
of these latter has nevertheless been maintained. It 
is true that the name of Italian ochre is often merely 
borrowed, for application to a product originating in 
some other country, varieties of terra di Siena, for 
instance, being put on the market that have actually 
been derived from deposits in Germany. 

As a result of this custom, certain names, such as 
terra di Siena, umbra di Roma, have become generic 
terms, and their use denotes, not an intention to suggest 
that the earth colours in question really come from 
Siena or the vicinity of Rome, but that the properties 
of the article are equal to those of the old-established 
colours of Siena or Rome. 

It would occupy too much space to go into an 
exhaustive description of all the native varieties of 
ochre, and would inevitably lead to a good deal of 
repetition. It will therefore be sufficient, for our 
purpose, to deal with only a few of them. 

The best -known ochres are those of Rome and Siena, 


the latter being frequently called, in commerce, by its 
Italian name, terra di Siena. 

Roman ochre forms yellowish-brown masses, of 
fairly fine texture and composed of ferric hydroxide 
and clay. They are put on the market both in the 
raw and calcined state. On calcination, the colour 
soon changes to red, and if carefully performed, the 
resulting colours have a very warm, fiery tone. 

Closely approaching Roman earth is the English 
ochre, which is worked more particularly in Surrey, 
and is not infrequently sold as Roman. In many 
deposits this English ochre occurs in such a high state 
of purity that the best pieces are picked out and sold 
without being even crushed or ground. The pieces 
of lower quality are very carefully ground and levigated, 
for the purpose of being calcined for the production of 
different shades, and then furnish highly prized colours. 

In point of chemical composition, the ochre family 
also includes terra di Siena, bole, umber and Cassel 
brown. These minerals, however, are not yellow like 
ochre, but brown, and will therefore be dealt with 
along with the brown earth colours. 


Products very similar, both in chemical composition 
and colour, to the native ochres can also be very simply 
and cheaply made by artificial means. Their prepara- 
tion may be particularly recommended to colour- 
makers who desire to turn out a wider range of iron 
pigments, but are not in a position to obtain natural 
ochres at a low price. 

In the manufacture of artificial ochre, an endeavour 
is made to imitate the natural processes which have 


led to the formation of ochre, and, of course, to avoid 
anything likely to hinder the production of a suitable 
colour earth, for example the presence of sand or a 
considerable admixture of extraneous minerals. 

As already mentioned, the chief impurities in natural 
ochres are clay and sand, both of which can be easily 
excluded during the manufacture of artificial ochre, 
or their amount controlled in such a manner that paler 
or darker products can be obtained at will, and the 
tone varied, in any desired manner, by calcination, as 
in the case of the native article. 

The raw material for artificial ochre is always a 
ferrous salt, which can be purchased in large quantities 
and at very low prices, namely green vitriol, which, 
in the pure state, consists of ferrous sulphate, FeSO 4 
-f 7H 2 O. This substance forms sea-green crystals, 
which are readily soluble in water and impart an 
objectionable inky flavour thereto. On exposure to 
the air, green vitriol turns an ugly brown colour, and 
is no longer completely soluble in water, passing gradu- 
ally into the condition of basic ferrous sulphate. This 
is because ferrous oxide is a highly unstable substance, 
which attracts oxidation and changes into ferric oxide. 
This latter, however, requires for the production of 
soluble salts a larger quantity of acids than does ferrous 
oxide, and therefore the oxidation of ferrous sulphate 
in the air leads only to the formation of salts that are 
imperfectly saturated with acid, namely basic salts. 

When a solution of green vitriol is left exposed to 
the air, basic ferric sulphate is also formed, which 
settles down to the bottom of the vessel as a rusty 
powder. If, however, a corresponding quantity of 
sulphuric acid be added to the solution at the outset, 
the resulting ferric sulphate remains in solution. 


On treating the green vitriol solution with one of 
caustic potash, caustic soda or quick lime, the ferrous 
oxide is thrown down as the corresponding hydroxide, 
forming a voluminous greyish-green precipitate. This 
hydroxide still possesses a great affinity for oxygen, 
and when the precipitate is brought into contact with 
air, its colour rapidly changes to a rusty red, through 
the transformation of the ferrous hydroxide into ferric 
oxide. The ferrous hydroxide can also be precipitated 
by alkali carbonates, the deposits behaving in exactly 
the same manner as that thrown down by the caustic 

Various methods can be adopted in the preparation 
of artificial ochre, the selection depending on the 
properties desired in the finished product. To obtain 
an ochre with particularly good covering power, the 
method must be different from that employed to furnish 
a cheap product, in which low price is more important 
than covering power. 

In the former case, the ferrous hydroxide is mixed 
with substances which, in themselves, possess fairly 
high covering power, such as chalk or white clay; in 
the second, gypsum, which is of low covering power, 
is used. 

The preparation of the cheapest kinds of artificial 
ochre will be described first, followed by that of the 
higher grades which belong to the most valued artists' 

For cheap artificial ochres, the ferrous hydroxide 
is thrown down by caustic lime from a solution of 
green vitriol. According as a lighter or darker shade is 
required, two to three parts of ferrous sulphate are 
dissolved in water, care being taken to select crystals 
of a pure green colour, since those that have a rusty 


look are only imperfectly soluble, because they contain 
basic ferric sulphate. 

The solution will always be cloudy, owing to the 
partial precipitation of the hydroxide by the lime in 
the water; but this is immaterial. For the precipita- 
tion, a milk of lime is prepared by slaking one to two 
parts of quicklime (according to the quantity of ferrous 
sulphate to be treated) in water, and stirring this up 
in enough water to make a thin milk. Care must be 
taken to exclude any large particles of lime, since these 
would find their way into the finished product and 
make the colour uneven. On this account, the milk 
of lime should be carefully strained through a loosely 
woven cloth or fine sieve, into the precipitation 

The ferrous sulphate solution is then poured in, the 
mixture being kept stirred, and an ugly, grey-green 
precipitate is produced, consisting of a mixture of 
ferrous hydroxide and calcium sulphate, the reaction 
being explained by the equation : 

FeSO 4 + Ca(OH) 2 - Fe(OH) 2 + CaSO 4 . 

The larger the amount of ferrous sulphate solution 
added to the milk of lime, the darker the resulting 
ochre. As soon as all the ferrous sulphate is in, the 
stirring is suspended, and the liquid is left until quite 
clear. The water is drawn oif through tapholes in 
the side of the vessel, care being taken not to disturb 
the fine precipitate, and fresh water is added, in which 
the deposit is stirred up and again left to settle down. 
This operation, which is once or twice repeated, is to 
wash the precipitate. 

When this object has been sufficiently accomplished, 
the mass is shovelled out of the vessel and spread 


thinly on boards, where it is left until the desired shade 
of colour has been attained, the colour changing quickly 
on exposure to air, owing to the oxidation of the ferrous 
hydroxide into ferric hydroxide. To ascertain whether 
oxidation is complete, a large lump of the mass is 
broken across ; and if it is of a uniform yellow-brown 
colour throughout, without being darker on the outside 
than in the middle, all the ferrous hydroxide will have 
been transformed into the ferric state. The product 
can now be dried at once, and when ground will be 
ready for sale. 

To obtain different varieties from the product, it 
is carefully heated (in a finely powdered condition) in 
shallow pans; but the operation needs caution, or the 
water in the gypsum present will be expelled, giving 
rise to drawbacks that are manifested when the colour 
is used. 

For instance, in mixing such a colour with water, 
the gypsum would again absorb water and cause the 
whole mass to set as a useless solid lump. Since 
gypsum parts with its water at a comparatively low 
temperature, it is better not to heat these cheap ochres 
at all, but to obtain the various shades by modifying 
the proportion of ferrous sulphate employed. 

Another defect of the ochres prepared by this method 
resides in the excess of lime present, it being impracti- 
cable to measure out the quantity of lime used with 
such accuracy that only just enough is taken to pre- 
cipitate the ferrous hydroxide, there being always a 
slight excess. This lime is transformed into calcium 
carbonate on the mass being exposed to the air, just 
as in the preparation of Vienna white; but as the 
saturation with carbon dioxide takes a considerable 
time, some of the lime remains in the caustic state 


and is liable to affect other colours that may be mixed 
with the ochre. 

An artificial ochre uniting in itself all the qualities 
of the natural product, and also capable of being shaded 
by burning, can be prepared in the following manner. 
An accurately weighed quantity of pure crystallised 
ferrous sulphate is dissolved in a definite amount of 
water, and the solution is treated with successive small 
portions of crude nitric acid, until all the ferrous oxide 
has been changed into the ferric state. The change 
can be detected by a very decisive test. If a liquid 
containing ferric oxide in solution is brought into 
contact with a solution of red prussiate of potash 
(potassium f erri cyanide) , no precipitate is formed in 
the absence of ferrous oxide, but only a brown colora- 
tion; whereas, if ferrous oxide is present, a beautiful 
blue precipitate is formed at once, the colour of which 
is so intense that very small quantities of ferrous oxide 
can be detected by this means. 

For the purpose now under consideration, the 
presence of small amounts of ferrous oxide in the 
solution is immaterial, because they are soon changed 
into ferric oxide on exposure to the air. It might, 
therefore, be asked, why take the trouble to oxidise 
the ferrous oxide by means of an agent involving 
expense, which could be saved by allowing the oxidation 
to take place in the air? 

The advantage, however, of the direct employment 
of a solution of ferric oxide is that it gives at once a 
colour that can be dried straight away; w r hilst at the 
same time the colour undergoes no change in drying, 
whereas it does when ferrous oxide solution is used. 

The method of producing ochres from this ferric 
solution varies according as the product is to be used 


without any further treatment than drying, or is to be 
modified by firing. 

In the former event, caustic lime is again used as 
the precipitant, but in only just sufficient quantity to 
throw down all the ferric oxide in the solution. This 
amount can be calculated exactly, 36-84 parts by weight 
of pure burnt lime being required for every 100 parts 
of pure ferrous sulphate taken. The actual quantity, 
whether larger or smaller, will depend on the relative 
purity of the sulphate and lime ; and this can readily 
be ascertained by a simple trial. 

The lime is used in the form of milk of lime, as 
already described. If lime alone is employed, the 
precipitate will consist of pure ferric hydroxide and the 
calcium sulphate thrown down at the same time. The 
resulting colour, when dried, will be an intensely brown 
mass, which can be used in place of the very dark 
natural ochres. 

In order to obviate entirely the disadvantages 
resulting from the presence of a large amount of caustic 
lime in the precipitate, fine levigated chalk or white 
clay is added in the preparation of the lighter shades 
of ochre, the addition being made as soon as the two 
ingredients have been brought into contact; and the 
mixture is thoroughly stirred, to ensure uniform 
admixture with the ferric hydroxide. The colour of 
the settled deposit will be lighter or darker in propor- 
tion to the amount of chalk or clay employed; and in 
this way the whole range of shades from pale yellow 
to bright brown can be obtained without the application 
of heat. 

Ochre that has been made with chalk is unsuitable 
for toning by heat, because this treatment would 
causticise the lime, and the ochre could not be mixed 


with other colours, since these would be affected by that 
substance. On the other hand, when white clay is 
used in preparing the ochre, the latter can be more 
easily toned by firing, provided care be exercised in the 
process. The ochre must be dried completely in the 
air, and either spread out in thin layers on iron plates, 
for the burning process, or else put into a drum, of the 
kind already described, in which the mass is moved 
onward by a worm. 

The clay remains unaltered in firing, but the gypsum 
parts with its water of crystallisation. In order to 
restore the latter, the ochre issuing from the drum is 
discharged direct into a vessel of water, in which it 
can be kept in constant motion by a stirrer. The 
water is soon warmed by the heat of the mass, and 
absorption by the gypsum proceeds at a rapid rate. 
When the whole charge has been fired and collected in 
the vessel of water, the stirrer is stopped and the 
precipitate dried, being then ready for use. 

In certain circumstances, ochre can be made by other 
methods. In large towns, ammonium salts are some- 
times obtainable at a moderate price, being manu- 
factured in large quantities as a by-product in gasworks. 
For our purpose, crude gas liquor might be used, since 
it contains ammonia for the precipitation of the 
ferric hydroxide. In most cases, however, this gas 
liquor contains only very small quantities of ammonia, 
and, therefore, in a works of any size, very large vessels 
would be needed for the production of a comparatively 
small quantity of ochre. On this account, preference 
is given to crude carbonate of ammonia, which is also 
obtainable at low prices. 

On bringing a solution of this salt into contact with 
one of ferric oxide, ferric hydroxide is precipitated, 


and the sulphate of ammonia resulting from the 
reaction remains in solution. By stirring white clay 
into the liquid at the same time, the ochre can be 
correspondingly lightened in shade. 

The precipitates obtained in this way can be dried 
at once, and converted into any shade obtainable 
with natural ochre, from brown to red, by strong 
firing. The sulphate of ammonia still remaining in the 
air-dried product is completely volatilised by the heat, 
and the resulting ochres are even superior to the 
natural varieties in beauty and permanence. 


In the manufacture of certain chemicals, substances 
of divergent composition are obtained which are sold 
under the name of ochre and are used as painters' 
colours. Whereas ochre, properly so-called, consists 
of either ferric hydroxide or ferric oxide in association 
with clay, lime, etc., the products now under considera- 
tion are basic ferric salts composed of varying quantities 
of ferric oxide in combination with certain proportions 
of sulphuric acid. 

These ochres are obtained as by-products in the 
manufacture of green vitriol from pyrites, and in alum 
manufacture ; and, according to their origin, they are 
classed as vitriol ochre, so-called alum sludge, and pit 
ochre. All the basic ferric sulphates of which they are 
composed form fairly large crystals, and, therefore, in 
most cases, the covering power is small. On this 
account the products are of low grade and are put 
on the market at low prices, for which reason they are 
largely used in making cheap paints. 

Vitriol Ochre. Commercial green vitriol is, for the 


most part, manufactured from native sulphides of 
iron. When many of these sulphides are piled in 
heaps and left to the action of the air, oxygen is gradu- 
ally absorbed and green vitriol is formed which is 
dissolved out by rain and is collected in large clarifying 

In the case of pyrites, however, the mineral must 
first be roasted in a current of air, since otherwise its 
conversion into green vitriol would only proceed in a 
very sluggish manner. In any event, the aqueous 
solution of ferrous sulphate has to be concentrated, 
by evaporation, to the point at which the green vitriol 
crystallises out. 

Both in the clarifying-tanks and still more so in 
the evaporating-pans, a rusty-looking sediment forms 
at the bottom, consisting of basic ferric sulphate. 
This originates in the partial oxidation of the ferrous 
oxide (first formed) while the pyrites is exposed to the 
air, and since the quantity of sulphuric acid present is 
insufficient to saturate all the ferric oxide, basic salts 
are produced. 

The yellow-brown sludge deposited in the pans 
during the concentration of crude green vitriol liquor, 
constitutes the product termed vitriol ochre, which 
contains varying amounts of ferric oxide, sulphuric 
acid and water, according to the quantity of ferric 
oxide resulting from the oxidation of the pyrites and 
the character of the latter, e. g, : 

* Ferric oxide .... 65-70% 
Sulphuric acid .... 14-16% 
Water 13-16% 

Although the colour of these ochres is not particularly 
handsome, they can be transformed, by firing, into 
colours of fairly good quality. As this subject will be 


more thoroughly gone into when dealing with the 
preparation of the red iron pigments, the applicability 
of these ochres will only be casually referred to here. 
During the burning process, these ochres, of course, 
part with the whole of their contained water; and by 
protracted, high calcination, the whole of the sulphuric 
acid can also be expelled, so that finally nothing but 
pure ferric oxide is left. 

Alum Sludge. Solutions of crude alum always 
contain a certain amount of ferric oxide which settles 
down at the bottom of the pans during concentration. 
This sludge, too, consists of basic ferric sulphate, but 
is inferior in covering power to vitriol ochre, the 
crystals being of coarser grain. On the other hand, 
the ochreous sediment from the alum concentrating- 
pans has the valuable property of being readily 
transformable into red-brown to pure red tones by 
burning. For this reason, particular attention has 
been devoted to this sludge in a number of alum works. 

Since the products are only of value when burned, 
and the shades thereby obtained are always red, they 
will be dealt with more fully along with the red earth 

Pit Ochre. Springs containing small quantities of 
ferrous sulphate and other salts are met with in many 
iron mines, but, in most cases, the amounts are too 
small for their recovery by artificial concentration to 
be contemplated. If, however, the conditions allow 
of the springs being easily diverted, they may often be 
utilised for the preparation of low-grade ochre. 

The chemical composition of these pit ochres varies 
considerably, and depends on the geological character 
of the locality. Water can only dissolve such minerals 
as occur in the form of fairly readily soluble com- 


pounds; and for this reason pit waters are always 
solutions of the metals which are found in the mine. 

The variety of compounds that may be present in an 
ochre can be seen from the subjoined analyses of 
ochres deposited from pit waters at Rammelsberg. 
As elsewhere, two distinct classes of ochre are met with, 
having a conchoid and an earthy fracture respectively. 
The latter usually contain rather more ferric oxide, 
and, in particular, a higher content of foreign sub- 
stances, the most important of which is quartz sand. 
In the Table, the ochres with conchoid fracture are 
marked A, and those with an earthy fracture, B. 

A. B. 

Ferric oxide . . . 68-75 63-85 

Zinc oxide . ... 1-29 1-23 

Copper oxide . . . 0-50 0-88 

Sulphuric acid . . .9-80 J 3'59 

Water . ... 15*52 18*45 

Clay and Quartz . . . 4-14 2-00 

The preparation of the ochre is a simple matter, 
consisting in collecting the mass and sorting out the 
loose, earthy portions of a pure yellow colour from the 
denser and darker parts. The former are dealt with 
separately, usually by a simple process of levigation, 
for the sole purpose of getting rid of the earthy matter, 
quartz sand in particular. 

The denser varieties require much more work, but 
yield a far superior product, which, by suitable treat- 
ment, can be converted into the finest grades of ochre. 
The first operation consists in a very careful crushing, 
and as the pieces are often very hard, they are treated 
in ordinary or stamp-mills, edge-runners being also 
employed with advantage. 

The product reduced by any of these means is passed 
through a number of sieves, to separate the fine 


particles from the coarse; and the finest dust is burnt. 
This last treatment causes a considerable loss in weight, 
both the accompanying water and most of the sulphuric 
acid being volatilised. However, since, as already 
stated, all varieties of ochre can be obtained, the 
process is consequently very remunerative notwith- 
standing the loss in weight it involves. 

Yellow Earth. From the particulars given in the 
general description of the earth colours, yellow earth 
may also be regarded, to some extent, as an ochre, 
but one containing a large proportion of foreign sub- 
stances. It might, however, be more accurately 
termed a clay contaminated with a considerable amount 
of quartz sand and a certain proportion of ferric oxide. 
The method of preparation is on the same lines as for 
ochre, but burning is never practised, nor is the treat- 
ment so careful as for the better grades of ochre, the 
low price of the colour making this unremunerative. 



THE number of minerals that can be directly used 
as red earth pigments is comparatively small, and by 
far the greater proportion consist of ferruginous colours, 
a few of which are obtained by the mechanical treat- 
ment of native iron ores or clays coloured red by ferric 
oxide, the majority, however, being formed by burning 
certain materials of another colour. To these belong 
nearly all the materials mentioned in connection with 
the ochres and the brown iron colours, together with 
a few by-products of the chemical industry. 

In addition to the foregoing, which have ferric oxide 
for their pigmentary principle, is the native mercury 
sulphide, occurring, as scarlet, crystalline masses, under 
the name of cinnabar (vermilion). The only reason 
for including natural vermilion with the earth colours 
is to make the list complete, the largest proportion 
of this pigment being prepared by artificial methods. 
The product sold as " Chinese " vermilion may, in 
former times, have really been introduced from China 
into Europe, and prepared there by grinding and 
levigating the best-coloured lumps of the natural 
cinnabar; but, at the present time, all the vermilion 
made in Euro peat least is from sulphur and mercury, 
by artificial processes, and the name Chinese vermilion 
is merely retained to designate a particularly fine grade. 


On the basis of occurrence and chemical properties, 
the red earths can be classified into several groups. 
The first comprises natural products requiring only 
mechanical preparation, such as the minerals known as 
hematite, micaceous iron ore, Elbaite, etc., and the 
special modification of red ironstone termed raddle. 
All these minerals consist almost entirely of ferric 
oxide in a pure state. The mineral, bole (red chalk, 
terra sigillata, Lemnos earth), is chemically allied to the 
ochres, being, like them, composed of alumina, fre- 
quently accompanied by lime and small quantities of 
magnesia, but differing in that ferric oxide is always 
present in bole, whereas the ochres always contain 
ferric hydroxide. 

The second group consists of the artificial reds 
obtained by burning or calcining raw materials, whose 
ferric hydroxide is more or less transformed by heat 
into ferric oxide, such as ( vitriol ochre, pit ochre and 
alum sludge. 

Of late years the artificial earth colours have attained 
a high degree of importance. They are obtained in 
large quantities in the manufacture of sulphuric acid 
from green vitriol. Formerly, it is true, they were 
also used as pigments under the name of caput mortuum 
or colcothar, but were not held in much esteem; and 
it is only within recent times that it has been discovered 
that these inferior by-products can be converted into 
very handsome and brilliant colours, which now form 
important articles of commerce. 


Bole, Lemnos earth, terra sigillata, etc., is, for the 
most part, a product of the decomposition of highly 


ferruginous minerals, and occurs, in the form of lumps, 
having a conchoidal fracture, in pockets or detritus. 
The lumps have a sp. gr. of 2-2-2'5, are Isabella brown 
to dark brown in colour, and give a slightly greasy- 
looking streak. There are two distinct varieties of 
bole : the one adhering firmly to the tongue, whilst 
the other lacks this property and, when placed in water, 
crumbles down to powder in emitting a peculiar noise. 

The composition of the boles varies, but all of them 
may be regarded as alumino ferric silicates combined 
with water. Most of the specimens examined from 
different deposits contain 24-25% of water, 41-42% 
of silica, and 20-25% of alumina, the remainder con- 
sisting of ferric oxide with small traces of manganese 

Some varieties, however, are exceptional and contain 
only 30-31% of silica and 17-21% of water, e. g. those 
from Orawitza and Sinope. Lemnos earth, the true 
terra sigillata, is mostly silica (66%) with 8% of water, 
and contains a smaller percentage of ferric oxide than 
the others. It is also of a distinct colour, lighter than 
the true boles and having a greyish or yellowish tinge. 

The behaviour of the different kinds on burning is 
just as diverse as their chemical composition. Whilst 
some kinds are infusible at even the highest tempera- 
tures, and merely change into hard, red masses; others, 
again, fuse at a moderate heat. This difference is due 
to chemical composition, those high in silica being 
generally less refractory than those in which alumina 

In order to render the boles suitable for painting, 
they are put through a somewhat different treatment 
than the other earth colours. The freshly dug material 
is first sorted, the uniformly coloured lumps of fine 


texture being set apart and suffused with water, with 
which they form a pasty mass of low plasticity, which 
is kneaded by hand to make it homogeneous, and is 
then stirred up with more water. When the lumps 
have distributed in the water, the latter is drawn off 
into a second tub, and the residue is stirred up with 
fresh water, the treatment being repeated until the 
effluent no longer shows any signs of colour. 

The liquid in which the finely divided bole is sus- 
pended is left to settle, and the bole subsides as a fine 
powder, which is dried to the condition of paste, 
pressed into moulds and dried completely. 

Owing to its low content of ferric oxide, the colour 
of bole is not particularly bright, but is very permanent 
a property equally shared by all the other ferric 
oxide pigments. 


In nature, ferric oxide forms extensive deposits, 
which, by reason of the light red colour characteristic 
of certain varieties of ferric oxide, are largely employed 
in painting. These colours may be classed among the 
oldest known to mankind, ferric oxide pigments having 
been used frequently in the most ancient paintings. 

The most important varieties of ferric oxide for our 
purpose are : iron glance, with its modifications, 
micaceous iron ore and frothy hematite ; red hematite, 
and raddle. 


This substance forms handsome black crystals of 
very high lustre, which, when small and scaly, con- 


stitute micaceous iron ore. Both, when rubbed down, 
furnish a dark red powder of no particular beauty. 
Micaceous iron ore forms the transition stage into 
frothy hematite, or iron cream, the sole difference 
being that the crystals of the latter are much smaller, 
and the scales finer, the iron-black colour passing 
gradually into cherry red. At the same time, the 
lustre, though still high, loses most of the metallic 
sheen exhibited by micaceous iron ore. 


The variety known as hematite or bloodstone, 
sometimes occurring as shiny nodules, is distinguished 
by its handsome red colour. Some of the lumps are 
composed of long, thin crystals grouped about a 
common centre so as to form a globular mass. Despite 
its bright colour, the hardness of hematite (between 
3 and 5) prevents it from being used as a pigment, 
the value of the product not being commensurate 
with the cost of reduction. 


There are numerous deposits of red ironstone, in 
the state of fine earth, where the operations of grinding 
and levigation have, to a considerable degree, already 
been carried out by Nature. These deposits form the 
mineral which, under the name of raddle, is often used 
as a pigment for ordinary paints. It may be con- 
sidered to have originated in the transformation of 
red ironstone, by the natural forces that can every- 
where be seen disintegrating rocks, namely water 
and frost, into a fine powder, which has been trans- 


ported, often over long distances, by water, and has 
finally settled down. 

In places where the process has been carried out in 
this manner, the raddle will be in a condition, as regards 
fineness of division and beauty of colour, that leaves 
nothing to be desired, and the material itself is ready 
for use as a very valuable pigment. Large deposits 
of this kind, however, are of rare occurrence; but 
there are plenty in which the ferric oxide is associated 
with varying quantities of clay, sand, and sometimes 

The conditions here are on all fours with those of 
clay, which, too, has been formed in a similar way. 
Pure clay, the so-called kaolin, is a highly valuable 
material, whereas ordinary loam highly contaminated 
clay is only of low value. In judging the quality of 
raddle as a pigment, the presence of impurities is of 
less account than their nature; and in some cases a 
very highly contaminated raddle may be worth far- 
more, as a pigment, than one containing only very 
small admixtures of extraneous substances. 

As stated above, the ordinary impurities in raddle 
are clay, lime and quartz sand. An admixture of 
clay, even if fairly large, is no great drawback, since 
the material can be used in its natural state, and also 
be toned by burning. Lime is less favourable, for 
though a calcareous raddle can be used as it is, the 
lime parts with its carbon dioxide on calcination, 
becoming changed into caustic lime and imparting 
to the product qualities which preclude its employment 
for a number of purposes, especially for mixing with 
delicate organic colours. 

The presence of quartz sand is immaterial when the 
raddle is to be burned, inasmuch as sand is unaltered 


by calcination. But it constitutes a drawback because 
it makes the fine raddle gritty and unsuitable for fine 
paint work. The only way to eliminate this impurity 
is by levigation an expensive operation which should, 
as far as possible, be avoided for these native ferric 
oxides, because they must be sold very cheaply, and 
have to compete with the large quantities of oxide 
obtained as a by-product of the chemical industry. 

The suitability of a given specimen of raddle for use 
as a pigment may be easily ascertained by weighing 
out exactly 100 grams and heating to about 120 C. 
The loss of weight will give the amount of water in 
mechanical retention. The residue is suffused with 
strong vinegar, and left for several days, being stirred 
at frequent intervals. The carbonates of lime and 
magnesia present will dissolve in the acid, the ferric 
oxide remaining untouched. The liquid is decanted, 
and the residue washed several times w r ith water and 
dried, the diminution in weight being a measure of 
the carbonates in the sample. If the vinegar has 
turned a yellow colour, the presence of ferric hydroxide 
in the mineral is indicated, this hydroxide being readily 
soluble in acetic acid. If the residue feels gritty, it 
contains quartz sand, the amount of which can be 
found with sufficient accuracy by levigating the mass 
and weighing the sandy residue after drying. 

Deposits occur, in many places, of a mineral similar 
to raddle, but formed under peculiar conditions. 
Thus, there are found, in the vicinity of brown-coal 
deposits that are rich in pyrites, earthy masses which 
are occasionally of a handsome red colour and consist 
of a variety of minerals admixed with a considerable 
proportion of ferric oxide. 

These masses probably originated in fires in the coal 


seams, whereby the pyrites became transformed into 
ferric oxide and basic ferric sulphate; and where the 
deposits are of sufficient size, they may be advan- 
tageously utilised in the production of cheap reds. 
In most cases, however, the minerals must be levigated, 
owing to the frequency with which they contain large 
proportions of extraneous minerals in a gritty condition. 


It has already been stated, in dealing with the 
yellow ochres, that these colours can be toned by 
burning, part of the ferric hydroxide losing its water 
and changing into red ferric oxide. The more severe 
the burning, the larger the amount of ferric oxide 
formed and the nearer the colour of the product 
approximates to red. According, however, as the 
original cchre was yellow or brown, the tone of the burnt 
colour will lie between orange and brownish red. If 
the heating be pushed so far as to transform all the 
ferric hydroxide into oxide, the red will come more 
and more into prominence in proportion to the amount 
of hydroxide in the original material. If the product 
consists entirely of ferric oxide, as is the case with that 
obtained, as a by-product, in the manufacture of English 
sulphuric acid, a pure red ferric oxide (caput mortuum, 
colcothar, English red, etc.) will be obtained. If the 
heating be increased above a certain point, the pure 
ferric oxide will change colour, assuming a brown to 
violet tone according to the temperature employed. 

(a) Burning in the Muffle 

Since, as a rule, the quantity of material treated 
in the preparation of these brown, violet to black ferric 



oxide pigments for the purposes of the painter on 
porcelain is not large, the same kind of muffle furnace 
(Fig. 29) as serves for making enamels can be used. 
The fireclay muffle M is inserted in a reverberatory 
furnace 0, with a good draught, and is raised to a 
white heat. The finely powdered material to be burned 
is spread out evenly on plates of sheet-iron or fire-clay, 

FIG. 29. 

and introduced into the white-hot muffle, where it is 
left for a period corresponding to the colour desired. 
To save time, the plates may be pre-heated in a second 
muffle arranged above the first. 

By this means a large range of tones can be obtained 
from one and the same material, by heating it to 
different temperatures; and the colours, so produced 
are distinguished, not only by their warmth of tone, 
but also by very high stability. In fact, they may be 


regarded as permanent, because very strongly calcined 
ferric oxide only passes very slowly into solution even 
under prolonged boiling in the strongest acids. Owing 
to this excellent property, which is equalled by very 
few other pigments, and the low cost of preparation, 
these colours deserve the most careful consideration 
by all manufacturers who are in a position to obtain 
suitable material in sufficient quantities. 

(b) Caput Mortuum, Colcothar 

Previous to the English method of making sulphuric 
acid by the oxidation of sulphur dioxide with nitric 
acid, this acid was manufactured by heating dehydrated 
ferrous sulphate (green vitriol) ; and even now, fuming 
sulphuric acid oil of vitriol, or Nordhausen sulphuric 
acid is largely obtained by the same process. 

When anhydrous ferrous sulphate, FeS0 4 , is exposed 
to a very high temperature strong white heat it is 
decomposed into sulphur trioxide, SO 3 , sulphur dioxide, 
SO 2 , and a residue, mainly composed of ferric oxide 
and a little basic ferric sulphate, which remains behind 
in the heating-pan. In fact, even at the highest 
possible temperatures obtainable in the furnaces used 
for the distillation of the green vitriol, it is impossible 
to recover the whole of the sulphuric acid, a small 
portion being tenaciously retained by the iron. 

This red residue is sold under various names 
colcothar, caput mortuum, English red, Indian red, 
etc. and is used as a low-grade pigment, and also as a 
polishing agent. The name caput mortuum is a 
survival from the time of the alchemists, and was 
probably applied to indicate a dead-burned product, 
from which all the active ingredients had been removed. 

Although, in former ages, this substance was held 


in low estimation as a pigment, attempts have been 
made in recent times to convert it, by suitable treat- 
ment, into a more valuable product ; and these 
attempts have been crowned with success, affording 
another instance of how a high commercial value can 
be imparted to a waste product by proper manipulation. 

(c) Calcining Ferric Oxide 

In order to obtain a series of tones of colcothar, it 
is subjected to repeated calcination, but not by itself, 
since it would require an extremely large quantity of 
fuel to effect any change of tone in view of the very 
high temperature the material has already been 
exposed to in the sulphuric acid plant. If, however, 
salt be added, then a variety of tones can be obtained 
without recourse to any particularly high temperature. 
It is frequently stated that the only effect of the 
presence of salt is to keep the calcining temperature 
uniform, inasmuch as the salt volatilises at a strong 
red heat, and when that temperature is reached, the 
whole mass cannot get any hotter until the whole of 
the salt has passed off, all the heat applied being 
consumed in transforming the salt into the state of 

As a rule, however, the amount of salt added does 
not exceed 6% of the weight of the charge to be 
calcined ; and this quantity does not seem to be 
sufficient to keep the temperature at a uniform level 
through the several hours required for the calcining 
process. The author is therefore of opinion that the 
salt also has a chemical action on the material during 
the calcination. 

As already mentioned, colcothar is by no means 
pure ferric oxide, but always contains basic ferric 


sulphate. Now, it is feasible that some reaction may 
take place between the basic sulphate and the sodium 
chloride at calcination temperature, with the formation 
of caustic soda, which, being a far more powerful base 
than ferric oxide, deprives the latter of sulphuric acid, 
sodium sulphate being formed. The chlorine of the 
salt combines with the iron to form ferric chloride, 
which volatilises at a glowing heat. 

According to this hypothesis, therefore, the addition 
of common salt in the calcination of colcothar is less 
for the purpose of maintaining a uniform temperature 
within certain limits than for decomposing the basic 
ferric sulphate present and inducing the formation of 
a product consisting entirely of pure ferric oxide. 
The various tones obtained are due to the varying 
length of exposure to the heat. 

The following method is pursued in the conversion 
of colcothar into iron pigments on a manufacturing 
scale. The crude colcothar from the sulphuric acid 
j)lant is ground, as finely as possible, in ordinary mills, 
and the resulting soft powder is intimately mixed with 
salt, 2, 4 or 6% being the usual proportions added. 
The calcination is ordinarily continued for six hours in 
the case of the mixture containing the largest amount 
of salt ; but only two hours, or even one, for the other 

The operation is carried on in earthenware pipes, a 
large number of which (up to sixty) are built into a 
furnace. The latter is fired very carefully, the tem- 
perature being raised only very gradually, since ex- 
perience has shown that much better coloured products 
are obtained in this way than by raising the mass 
quickly to a high temperature. 

When incandescent ferric oxide is allowed to cool 


down with unrestricted access of air, the colour is not 
nearly so bright as when air is excluded during the 
cooling. Since air has no action on ferric oxide, this 
remarkable phenomenon cannot be due to the presence 
of the air, but probably to the influence exerted by the 
rapid change of temperature on the arrangement of 
the finest particles of the oxide. Nevertheless, some 
manufacturers hold that rapid cooling, with restricted 
access of air, improves the colour. 

To exclude air from the ferric oxide during calcina- 
tion, the open ends of the pipes are flanged and covered 
with close-fitting plates, which are luted with clay. 
The expansion of the internal air as it grows hot would 
burst the pipes unless a means of escape were provided, 
which consists in leaving small vent holes in the cover 

As previously mentioned, calcined ferric oxide is 
very inert, chemically, so that, when the calcination 
has been strong, prolonged boiling with the most power- 
ful acids is needed to bring the oxide into solution. 
If the heating has been continued up to the strongest 
white heat, and the ferric oxide maintained in that 
condition for several hours, even hot sulphuric acid 
will have only a slight effect on the oxide, and the only 
way to make it more readily soluble is by fusion with 
potassium bisulphate. 

Now indifference to chemical action is just the 
property required of a pigment for fine work; and in 
this respect, the ferric oxide colours are superior to all 
others. The gradations of tone that can be obtained 
from ferric oxide by varying the calcination are very 
numerous, comprising all between iron red, red-brov/n 
and pure violet. 

The author has tried heating ferric oxide for a 


considerable time at a very high temperature, equiva- 
lent to the strongest white heat, and obtained a product 
which was no longer pure violet, but had a decidedly 
blackish colour. Perhaps, by greatly prolonging the 
heating, it might be possible to get a pure black; but, 
even if this were so, the matter would be of no special 
interest, because black pigments for paints can be 
prepared in a much cheaper manner. All that would 
be accomplished would be the proof that ferric oxide 
actually undergoes an extensive molecular modification 
when heated. 


Alum is manufactured from alum shale and alum 
earth, the former being a carbonaceous clay shale 
interspersed with pyrites, and the latter a clay charged 
with pyrites and bitumen. The raw materials are left 
in heaps for several years, the pyrites being thereby 
oxidised with formation of free sulphuric acid and 
ferrous sulphate. This free acid reacts further on the 
clay, which it transforms into sulphate of alumina; 
and by leaching the heaps with water, a solution is 
obtained which contains the sulphate of alumina and 
the ferrous sulphate. On the liquor being concen- 
trated, a basic ferric sulphate is deposited, which is 
worked up into red pigment. 

For this purpose it is first levigated in a special 
manner, the sludge from the pans being placed in a 
large vat, suffused with water, and kept in slow circula- 
tion by stirrers, which distribute the particles in the 
water, forming a turbid liquid. This liquid is con- 
ducted into a. gently sloping shute, the sides of which 
are perforated with openings at certain intervals, to 



allow part of the liquid to run off into large collecting 
vessels underneath. 

The heaviest of the suspended particles settle down 
first and are flushed out by the water escaping through 
the first opening. The finer the particles, the longer 
they remain in suspension, so that the liquid escaping 
through the last holes carries off only a very fine 
powder. The liquid collected in the different vessels 
is allowed to subside and is then drawn off from the 

FIG. 30. 

firm deposit. The operation is repeated with fresh 
quantities of sludge until sufficient sludge has been 
collected for further treatment. The collecting vessel 
furthest away from the intake of the shute contains 
the finest levigated material, and this is used for making 
the best ochres. 

The levigated mass is dried in a very simple manner, 
being usually spread out on boards, which are exposed 
to the air in open sheds, covered with a roof to keep 
out the rain. Here the sludge is left until it forms a 
pasty or earthy mass, and is then calcined. 



The best calcining furnace is of the type used for 
colcothar; but the pipes must be connected to an 
exhaust pipe for carrying off the vapours disengaged 
during calcination. 

However, since alum manufacturers do not usually 
go in for making the highest-grade pigments, simpler 
calcining furnaces are used, consisting of reverberatory 
furnaces in which the heating-gases are allowed to act 
directly on the materials of the charge. A front 

FIG. 31. 

elevation and section of such a furnace are shown in 
Figs. 30 and 31. The furnace is constructed with 
several arches, one above another, marked c, k, d. 
The charge is introduced through the openings b and 
b'. The furnace chamber is at a, and the ashpit at g. 
The gases of combustion flow over the charge on the 
hearths of the several arches and escape, at the top, 
into the stack, along with the acid vapours liberated 
from the glowing mass. 

The further the hot gases get away from the fire, 
the cooler they become, and therefore the less strongly 
heated the charge on the upper hearths. Consequently, 


the resulting product (ferric oxide) from the different 
stages of the furnace differs in colour; and a number 
of gradations can be obtained by blending. The ferric 
oxide pigments prepared in this way are not pure 
oxide, but also contain small quantities of sulphuric 
acid and metallic oxides which were present in the 
original crude sludge. However, by reason of the 
simple process of preparation employed, these pigments 
are usually sojd at lower prices than those from colco- 
thar; and for less fine work they are excellent. 



IN point of chemical composition, the majority of 
the brown earth colours are closely allied to the reds, 
both kinds containing ferric oxide. The main difference 
consists in that, in the brown earths, the ferric oxide 
is combined with water to form ferric hydroxide. 

Many of the brown earth colours, however, are of 
entirely different chemical composition, and either 
consist mainly of organic matter derived from the 
decomposition of plants and therefore very similar 
to brown-coal or peat or else contain varying quanti- 
ties of inorganic substances mixed with these dark- 
coloured organic decomposition products. 

The brown earth colours form a highly important 
group, some of the members of which are used in the 
finest paintings, and, for certain purposes, cannot be 
replaced by other pigments. Those containing ferric 
hydroxide are found though not very frequently in 
natural deposits, the most celebrated being the terra 
di. Siena, occurring in the vicinity of that city. 


This highly renowned pigmentary earth is found in 
deposits, and, in the crude state, forms dark brown 
masses which are devoid of lustre, crumble readily 
between the fingers, have a smooth conchoidal fracture 


and absorb water with avidity, in consequence of which 
property they adhere to the tongue. Their chief 
chemical constituent is ferric hydroxide, with which, 
however, variable quantities of sand, clay and ferric 
oxide are admixed. These admixtures cause a con- 
siderable divergence in the colour of the earth, ranging 
from pure brown to reddish -brown, and, in the case 
of very impure lumps, to an unsightly yellow-brown. 

Mineralogically, terra di Siena is often regarded as a 
distinct species which, according to the results of 
analysis, must be considered, not as ferric hydroxide, 
but as ferric silicate combined with water. Sometimes, 
a portion of the ferric oxide is replaced by alumina, so 
that the percentage composition of the mineral becomes 
approximately: ferric oxide, 66%; silica, n%; 
alumina, 10% ; and water, 13%. The hardness of this 
mineral is 2*5, and the sp. gr. 3-46. 

The method of formation of terra di Siena was 
probably on the same lines as that already described 
in the case of ochre, namely by the breaking down of 
minerals in this case brown ironstone and natural 
levigation, the powder being deposited in places where 
the water containing the ferric hydroxide in suspension 
came to rest and allowed the solid particles to settle 

The best lumps of terra di Siena in point of purity 
and colour can be used as pigments without any 
preparation; but in most cases the earth is lightly 
calcined, in order to improve the colour. This treat- 
ment enables a whole series of tones, from pure brown 
to the brightest red, to be obtained. The stronger the 
heating, the more water expelled from the hydroxide, 
and consequently the closer the approximation of the 
colour to that of ferric oxide. 


The pigments met with in commerce as terra di 
Siena can also be prepared artificially, by making ferric 
hydroxide and heating this, when dried, until the 
requisite tone is attained. For this purpose, ferrous 
oxide is precipitated from green vitriol and exposed to 
the air, under which conditions it is rapidly transformed 
into ferric oxide, and the greyish-green colour of the 
mass changes to brown. Lighter tones can be obtained 
by the addition of inert white substances; and, in 
other respects, the method of preparation is the same 
as that of artificial ochre. 

These pigments are sold under various names, the 
dark shades, between pure brown and red brown, being 
usually called terra di Siena or mahogany brown, 
whilst the paler sorts are sold as satinober more 
correctly satin ochre, golden ochre, etc. Other pig- 
ments, chemically allied to the ferric oxide or ochre 
pigments, are sometimes found on the market under 
various and entirely arbitrary names. 

It may be pointed out that the greatest confusion 
exists in the nomenclature of pigments, to such an 
extent that, in many cases, neither the chemist nor 
the manufacturer knows precisely what pigment is 
implied by a given name. The confusion is still further 
increased by the use of names taken from different 


Umber, properly so called also known as Turkish, 
Cyprian or Sicilian umber, from the country of origin 
derives its name, according to some authorities, from 
the province of Umbria (Italy), where a brown earth 
is found, though others ascribe it to the Latin " umbra " 


(shade) because of the pigment being used for painting 

True umber is an earthy mass of fine texture and 
liver-brown colour, merging into chestnut in some of 
the lumps. Chemically, it consists of a double silicate 
of iron and manganese combined with water, a portion 
of these metals being usually replaced by alumina. 
The greater hardness (1*5) and higher specific gravity 
(2*2) of true umber in comparison with Cologne earth 
(which is quite arbitrarily termed "umber-"), form a 
ready means of differentiation between the two. 

According to Viktor Merz, the umber found in 
Cyprus consists of : ferric oxide, 52% ; manganese 
oxide, 14*5%; and alumina, 3%; and is, possibly, 
merely a mixture of clay with hydroxide of iron or 
manganese. An umber examined by Klaproth con- 
tained 13% of silica, 5% of alumina, 48% of ferric 
oxide, 20% of manganese oxide and 14% of water. 

The tone of umber can be modified, in the direction 
of red, by calcination, but this process is seldom 
employed, the dark brown shade of this colour being 
the one most appreciated. 

In some parts of northern German} 7 , Thuringia in 
particular, the iron mines contain smaller or larger 
pockets of ferric hydroxide, of a fine earthy texture, 
from which umber is prepared, by levigation and 
calcination. The product is sold under various names : 
chestnut brown, wood brown, mahogany brown, bistre 
flea brown, roe brown, according to the shade of the 
calcined pigment. 

A mineral (" siderosilicate," according to Von Walter- 
hausen) composed of ferric silicate, and approximating 
in this respect to terra di Siena, is found in the neighbour- 
hood of Passaro (Sicily) in deposits of tuff. It forms 


masses which are transparent at the edges and are 
usually liver-brown to chestnut in colour. The hard- 
ness of the mineral is 2*5, the sp. gr. 2*713, and the 
average chemical composition: silica, 34%; ferric 
oxide, 48-5% ; alumina, 7-5% ; and water, 10%. 

The foregoing are only a few examples of brown or 
red-brown earth colours. In all these minerals the 
pigmentary principle is iron, in combination either with 
oxygen alone (ferric oxide), with oxygen and water 
(ferric hydroxide), or silica compounds (ferric silicate), 
and always associated with certain quantities of other 
metallic oxides, especially alumina and manganese 
oxide. Although but few of these minerals have 
gained any special reputation as pigments, there is 
no doubt that similar minerals, which are certain to 
occur in or near many deposits of iron , ores, could 
equally well be used for that purpose. There is no need 
to emphasise that the discovery of such a mineral 
would be a very valuable find, and that the products 
obtainable therefrom could be utilised to great 

The testing of a mineral for its suitability as pigment 
is a very simple matter, all that is required being to 
subject a small quantity to the same treatment that 
is applied to the earth colours on a large scale. For 
this purpose a few pounds of the mineral are levigated, 
and the residue is dried. To ascertain the tones 
obtainable by calcination, small samples of about 
100 grms. are placed in crucibles, and gradually 
heated in a furnace. When the masses have attained 
a sufficient temperature, the samples are taken out of 
the furnace, at intervals of ten minutes, and left to 
cool. It will then not be difficult to decide whether 
the mineral is at all suitable for the purposes of the 


colour-maker; and if so, these tests afford at once an 
indication of the temperature and time the mineral 
must be heated in order to obtain pigments of definite 


The application of the term " umber " to this earth 
can only have been based on a certain similarity in 
colour to true umber. In chemical composition, how- 
ever, the two are quite different, Cologne earth really 
consisting of a mixture of humic substances. It is 
well known that the rotted wood found in the interior 
of decaying trees is often a handsome brown colour; 
and all woody matter, after lying a very long time, 
finally acquires this colour, owing to the transformation 
of the wood into dark-coloured compounds richer in 
carbon. This effect can be seen on the large scale, in 
Nature, in the case of coal, brown coal and peat. 

Now Cologne earth consists of a brown-coal mould, 
dark brown in colour, of earthy character and of such 
low cohesive power that it crumbles with ease. Owing 
to this character, Cologne earth can be easily ignited 
by the flame of a candle, and then burns with a strong, 
smoky flame, leaving very little ash and disseminating 
the peculiar bituminous smell given off when brown 
coal is burned. 

The geological characteristics of Cologne earth enable 
one to conclude that, where similar conditions prevail, 
materials of analogous nature may be discovered. This 
earth is found embedded in a deposit of brown coal, in 
which it forms pockets, and occasionally large bodies. 
Now, brown-coal deposits of enormous extent occur in 
very many localities, as for instance in Upper Austria 


and in Bohemia; and many of these mines are sure 
to contain pockets of brown-coal mould, which have 
perhaps been overlooked, but might very well be 
utilised in the preparation of colours of very similar 
character to Cologne earth. 

The preparation of this material is very simple. 
The earth coming from the deposits is put through a 
simple levigation treatment which leaves, as residue, 
lumps of semi-decomposed wood, mineral admixtures, 
sand, etc. The levigated earth is sold in the form of 

Cologne earth comes into the market under various 
other names, such as : umber, Cassel brown. Spanish 
brown, etc. 

The fiery brown which was so greatly preferred by 
the famous painter Van Dyck, and named Vandyke 
brown after him, was of very similar composition to 
Cologne earth, and is said to have been obtained from 
a deep brown peat earth. The Vandyke brown of the 
present day, however, is almost invariably a ferric 
oxide pigment, toned to the proper shade by suitable 


As a natural product, which can be used as a painters' 
colour without any special preparation, asphaltum 
(bistre, bitumen) may also be classed among the earth 
colours. Chemically, it is composed of hydrocarbons 
of various kinds, and is thus similar to tar; in fact, 
asphaltum may also be regarded as a natural tar 
resulting from the decomposition of various orgaric 
substances. Many deposits of this mineral are known, 
and two of them are particularly celebrated : those 


of the Dead Sea, in Syria, and the Lake of Asphalt, in 
Trinidad. Both deposits consist of craters filled with 
water on which the asphaltum floats in large cakes. 

Several kinds of asphaltum are met with in com- 
merce, ranging in colour from brown to black. The 
preparation of the material as a pigment is confined to 
grinding the mass, which is always of a low degree of 
hardness. Being readily soluble in oil of turpentine 
and then furnishing the most beautiful brown tones 
when laid on thinly, the pigment is usually sold in this 
condition, although it is also ground in oil for the same 

Finally, it may be mentioned that various useless 
materials can be transformed, by suitable treatment, 
into brown pigments closely resembling Cologne earth 
and applicable to the same uses. Such pigments can 
be prepared from brown-coal slack (from inferior brown 
coal) or bituminised wood a variety of brown coal 
looking like charred wood by treating these materials 
with a lye made from wood ashes and lime, and washing 
and drying the residue. 



ALTHOUGH the number of green-coloured minerals 
is large, but few of them are suitable for painters' 
colours, because they occur so rarely in Nature that 
their employment for this purpose is out of the question, 
more especially since a very large number of green 
pigments can be obtained by artificial means. The 
most important of the earth colours in this category 
are Celadon green, or green earth, and malachite green 
the latter, however, less so, because the substance of 
which it is composed can be prepared artificially. 


This mineral is of a peculiar green colour, and the 
name " Celadon green " has been universally adopted 
in the nomenclature of colour shades. Green earth 
occurs native in many places, being the decomposition 
product of an extensively distributed mineral, augite, 
crystals of which are found in many of .the deposits. 
The green earth of Monte Valdo, on Lake Garda (Upper 
Italy) has been used for a very long time as a pigment. 
It is chiefly prepared in Verona for distribution in 
commerce, and from this circumstance has acquired 
the name " Verona green," or " Verona earth." The 
earth is also found in Cyprus and Bohemia, where it 




frequently occurs as the decomposition product of 
basaltic tuff. However, whether obtained from Monte 
Valdo or elsewhere, the product is always placed on 
the market as Verona earth. 

Native green earth is always tough, mostly occurring 
in amygdalous lumps, but occasionally in the crystalline 
form of augite. It has a fine-grained fracture, a hard- 
ness between i and 2, and a sp. gr. between 2*8 and 2*9. 
The colour is not always quite uniform, pure lumps 
having the characteristic Celadon green appearance, 
whilst impure lumps are olive green to blackish green. 
In chemical composition it is chiefly ferrous silicate, 
and this compound must be regarded as the actual 
pigmentary principle of green earth. In addition, it 
contains varying quantities of other compounds which 
influence the depth of shade of the product. 

Verona earth chiefly consists of ferrous oxide in 
combination with silica; alumina, magnesia, potash, 
soda and water being also present. Analysis shows it 
to contain : ferrous oxide, 21% ; silica, 51% ; magnesia, 
6%; potash, 6%; soda, 2%; and water, 7%. 

The green earths from Gosen, Atschau and Mannels- 
dorf , near Kaaden (Bohemia) and the Giant's Causeway 
(Ireland) have the following composition : 

^aden. 1 ^ 


4 r 56-4 

Alumina . 

3 2-1 

Ferrous oxide 

23 5'i 

Ferric oxide 




Magnesia . 

2 5'9 


3 8'8 

Carbon dioxide 






On account of the large quantity of mechanically 
associated water, freshly dug green earth is greasy in 
character, like wet clay. In partial drying, most of 
this water evaporates, the mass becoming earthy and 
adherent to the tongue. Sometimes the colour is an 
ugly brownish -green, owing to the presence of a con- 
siderable amount of ferric oxide formed as the result 
of changes set up by exposing the mineral to the air. 
Ferrous oxide is a very unstable compound, having an 
energetic tendency to combine with more oxygen and 
thus undergo transformation into ferric oxide ; so that 
when green earth is left in the air for a long time, a 
considerable proportion of its ferrous oxide is oxidised 
to ferric oxide, the mass thereby assuming the brown 
tone in question. 

Such an unsightly product can, however, be con- 
verted, by simple treatment, into one of very bright 
and handsome appearance ; and it is this possibility that 
first enabled green earth to attain importance as a 
painters' colour. Formerly it was only used as a 
material for common work, being added to whitewash 
or employed for indoor paints. 

When the crude green earth is treated with very 
diluted hydrochloric acid, the compound of ferrous 
oxide and silica is left intact, but most of the extraneous 
admixtures are removed. Ferric oxide, in particular, 
passes into solution, and the calcium carbonate largely 
present in some kinds of green earth is also dissolved. 
After prolonged contact with the crude earth, the 
acid liquor takes on a brownish coloration from the 
dissolved ferric oxide. Since the presence of iron 
salts has no influence on the purification of the green 
earth, the most impure, highly ferruginous hydrochloric 
acid can be used, and the liquor can afterwards be 


employed in the preparation of artificial ochre by 
leaving it in prolonged contact with any strongly 
ferruginous mineral, such as brown ironstone, which 
neutralises the surplus acid. This liquor is then 
precipitated by lime, alkali, etc., the resulting deposit 
consisting mainly of ferric hydroxide, the further treat- 
ment of which is conducted exactly as described in 
dealing with the preparation of artificial ochre. 

The treatment of the crude earth is best carried on 
in the same vessels as are to be used in the subsequent 
levigation process. After the acid liquor has been 
drawn off, the earth is brought into contact with water, 
stirred up well, and the w r ater run off, by opening tap- 
holes in the side of the vessel, into sett ling- tanks, where 
it is left until all the suspended matter has completely 

The colour of green earth can also be toned by the 
addition of yellow ochre, thus producing a range of 
greens with a yellowish tinge. These lighter shades, 
however, are seldom met with in commerce, the trade 
judging the quality of green earth more particularly 
on the depth of colour. 

Green earth is a valuable pigment for all kinds of 
painting, on account of its extreme permanence. It 
may be applied directly over lime without suffering 
any change, whereas most of the cheap green colours 
are destroyed in like circumstances. This behaviour 
renders green earth specially valuable in fresco work, 
although it is also largely used as an oil colour. 

Augite is of frequent occurrence in volcanic districts ; 
and in such localities, deposits of green earth are certain 
to be found. The test for the suitability of a green 
earth consists mainly in treatment with dilute hydro- 
chloric acid. If the mineral assumes a handsome green 


tone, it will generally form a useful pigment. The 
test may be supplemented by applying the colour to 
a fresh coating of whitewash, under which conditions 
it should remain unaltered. 


A product sometimes put on the market as green 
earth or green ochre has nothing beyond its name in 
common with green earth properly so called, except a 
certain similarity in colour. This pigment is prepared 
by mixing yellow ochre to a thin pulp with water and 
adding about 2% (of the weight of ochre) of hydro- 
chloric acid. After a few days, a solution of 2 parts of 
yellow prussiate of potash is added, and if the liquor 
still gives a precipitate when tested with ferrous 
sulphate, this last-named salt is added so long as such 
a precipitate continues to form. 

The deposit is washed, and dried in the ordinary 
way. When the right proportions have been taken, a 
pigment is obtained that coincides fairly in point of 
tone with true Verona earth. It is, however, inferior 
in point of permanence, the Berlin blue present being 
somewhat unstable and decomposing very quickly 
when brought into contact with lime. The reaction 
taking place in the production of so-called " artificial 
Celadon green " is that the hydrochloric acid used 
dissolves ferric oxide from the ochre, the addition of 
the yellow prussiate of potash then forming a blue 
precipitate of Berlin (Paris) blue which, in, conjunction 
with the yellow of the ochre, gives a green-coloured 



Although the pigment sold under this name is nearly 
always an artificial product, it cannot be omitted from 
a work dealing with the earth colours, because, in 
former times, it was prepared exclusively from the 
mineral malachite. Owing to the fact that artificial 
malachite green is one of the cheapest of colours, the 
troublesome work involved in the mechanical prepara- 
tion of the native pigment has been almost entirely 
abandoned, and the malachite itself is now utilised to 
greater advantage as a source of copper. 

Malachite green (or mountain green) is found in nearly 
every case where copper ores exist, and is still though 
very rarely indeed prepared, in a few places, from the 
mineral, the dark-coloured lumps being picked out 
because the lighter-coloured ones would furnish much 
too pale a colour. 

The treatment of malachite for the preparation of 
pigment presents certain difficulties owing to the com- 
parative hardness (3*5-4) of the mineral, which is also 
rather heavy (sp. gr. 3*6-4-0). On the large scale, the 
selected mineral is first put through a stamping-mill, 
and then ground, very hard stones being required for 
this purpose. The fine product from this (usually wet) 
process is levigated and dried. 

The pit water of some copper mines contains certain 
quantities of blue vitriol (copper sulphate) in solution ; 
and such pit water is generally treated for the recovery 
of a very pure form of copper, the so-called cementation 
copper. The liquor might also be worked up into 
malachite green, by collecting it in large tanks and 
precipitating the dissolved copper oxide with milk of 
lime, the bluish-green deposit separating in association 


with gypsum being transformed into a light malachite 
green by washing and drying. A darker green, free 
from gypsum, could be prepared by using a solution of 
carbonate of soda as precipitant. 

Neither the native nor the artificial malachite green 
is particularly handsome in colour; and both possess, 
in addition, the unpleasant property of gradually going 
off colour in the air, all the copper compounds being 
quite as sensitive to sulphuretted hydrogen as those of 
lead, and finally turning quite black under the influence 
of that gas. 



ONLY three minerals are known to be suitable as 
pigments ; and indeed, at present, only two, the third, 
lapis lazuli, being now of merely historical interest. 
Nowadays, no one would think of using this rare and 
expensive mineral as a pigment, since ultramarine, 
which has the same pigmentary properties,, is extremely 
cheap, whereas the pigment from lapis lazuli was 
worth its weight in gold. The only two blue earth 
colours of any interest at present are malachite (copper) 
blue, and the blue iron earth Vivianite; and even 
these, though by no means rare, are little used, since 
artificial blues are now made which are far superior 
in beauty and can be obtained so cheaply that the 
natural pigments are put out of competition. 


Lazulite and malachite (mountain blue) are of 
frequent occurrence in copper mines, and the former 
is distinguished by its beautiful azure blue colour, 
which, however, suffers considerably when the crystals 
are reduced to powder. Both minerals are very 
similar in chemical composition, and consist of cupric 
carbonate. The formula of malachite is 2CuOCO 2 + 
H O, that of lazulite being 3CuO(CO 2 ) 2 + H 2 O, so that 



the only difference between them is that of the relative 
proportions of the substances in combination. Lazulite 
is also rather hard (3- 5-4*0), but owing to the small 
size and brittle character of the crystals it is not very 
difficult to pulverise. In the air, malachite blue 
behaves in much the same way as malachite green, 
turning black in presence of sulphuretted hydrogen. 

Malachite blue is chiefly used for indoor work, and 
also as a water colour ; but it is always rather pale and 


This mineral also termed blue ochre is a trans- 
formation product of various iron ores, and occurs 
native as fairly extensive deposits in some places, 
especially in peat bogs. It forms ill-defined crystals, 
which are of a low degree of hardness (2*0) and vary 
in specific gravity between 2'6 and 2*7. The colour 
of the freshly won mineral is whitish or pale blue, but 
soon changes to a dark blue in the air, owing to the 
oxidation of the ferrous phosphate, originally present, 
into ferric phosphate. 

Vivianite can be transformed into a pigment by a 
simple process of crushing and levigation; but the 
product is never very handsome, and, at best, is only 
suitable for quite common paint work, though character- 
ised by considerable stability. 



ONLY two minerals are known that can be used as 
black earth colours, namely black chalk or shale black, 
and blacklead or graphite. Whereas the former of 
these is of merely subordinate importance, most of 
the black chalks being prepared artificially, graphite 
is all the more so because it is employed, not only as 
the sole material for lead pencils, but also for making 
graphite crucibles, as blacklead stove polish, as a 
lubricant, etc. One of its numerous applications is 
in connection with the electro deposition of metals, its 
high electrical conductivity causing it to be used for 
coating the interior of the moulds in which this deposi- 
tion is effected. 


Graphite, also known as plumbago or blacklead, 
consists of carbon. It is usually spoken of as pure 
carbon, but from a very large number of carefully 
conducted analyses, it would appear that native 
graphite is never quite pure, even the finest grades of 
the mineral containing 96-8% of carbon at the most. 
The accompanying substances which in some cases 



form nearly 50% of the whole are of divergent com- 
position and consist of iron, silica, lime, magnesia and 
alkalis. Even the combustible constituent of graphite 
is not pure carbon, but always contains a certain 
though small proportion of volatile substances. These 
slight traces of volatile matter are of considerable 
importance in connection with the hypothesis on the 
origin of the mineral. 

Contrary to the old idea, it is now almost universally 
considered that, instead of being of volcanic origin, 
graphite consists of the remains of long-dead organisms, 
and in this respect is closely related to coal. This 
hypothesis, however, fails to explain one point, namely 
the crystalline nature of graphite ; for even anthracites, 
which form the oldest coals known to have had their 
origin in the decomposition of organic substances, do 
not reveal the faintest traces of crystalline structure. 
The upholders of the theory that graphite was formed 
by the action of plutonic forces adduce, in support, 
the fact that graphite can actually be produced, in 
certain chemical processes, at high temperature. 
Molten cast-iron in cooling causes the separation of 
carbon in the form of graphite ; and the same substance 
is also formed, in large quantities, in gas retorts, 
through the decomposition of certain carbonaceous 
compounds when brought into contact with the 
glowing walls of the retorts. Recent investigations, 
however, have shown that the temperature necessary 
for the transformation of non-crystalline carbon into' 
crystalline graphite is by no means so high as was 
formerly supposed ; and it is now known that the change 
takes place at as low as red heat. Possibly the two 
theories could be reconciled by the assumption of a 
very old coal such as is found, for instance, as anthra- 



cite in many parts of the world being so strongly 
heated, by plutonic action, as to change into 

Native graphite crystallises in the form of hexagons, 
mostly tabular ; but really well -developed crystals are 
of extremely rare occurrence, and by far the greatest 
quantities of this mineral are found in the condition 
of dense lumps, in which only the crystalline structure, 
and not any decided crystals, can be discerned. The 
hardness of the mineral fluctuates within fairly wide 
limits, ranging from 0-5 to ro. The sp. gr. averages 
r8oi8-r844, but, in the case of impure lumps may 
increase to 1-9-2-2. 

The following analyses will give some idea of the 
considerable divergence existing between graphites 
from different deposits : 

Ash . 







Carbon . 

Water (chemically combined) 










Lime . 

Ferric oxide 


Water and volatiles 



J 7-34 











Carbon .... 




Ash .... 







Carbon .... 




Silica .... 




Alumina .... 




Ferric oxide 


Manganese protoperoxide . 
Lime .... 








Sulphur .... 


Loss on incineration . 



Of these Styrian specimens, Nos. 1-4 are crude 
kinds, of sp. gr. 2*1443; No. 5 was levigated in the 
laboratory, and No. 6 was levigated from an inferior 
quality at the mine. 

According to the character of the crystalline 
structure, the colour of graphite varies, but is mostly 
deep black. Very pure specimens, such as the beau- 
tiful graphite blocks (from the renowned Alibert 
graphite mines in Siberia) which, as a rule, are only 
to be seen in exhibitions and mineralogical collections, 
have the appearance of unpolished steel or white pig 
iron (spiegeleisen) . The most important property of 
native graphite is its low hardness and cohesion, in 
consequence of which it leaves a streak when drawn 
over the surface of paper. 

Graphite seems to be of frequent occurrence all over 
the world, though only few deposits are known which 
yield a product that is suitable for all the purposes to 
which graphite is applied. 

In European countries, Austria is particularly rich 
in graphite; and very large deposits of this mineral 
are found in Bohemia. Considerable deposits also 
occur in Bavaria, where they have long been worked. 


English graphite is celebrated for its excellent quality. 
All these European deposits, however, are surpassed, 
both in extent and in the quality of their products, by 
those discovered in Siberia, the largest being that 
producing the aforesaid Alibert graphite and situated, 
near the Chinese frontier, in eastern Siberia. At one 
time, America imported all her blacklead pencils from 
Europe, having, at that period, no known graphite 
deposits furnishing a suitable product. At present, 
however, deposits of this kind have been found in 
California, and there can be little doubt but that many 
others of this valuable mineral remain to be discovered 
in that enormous continent, the geological investigation 
of which is still far from being complete. 

The graphite of some deposits is so highly con- 
taminated by extraneous minerals that it cannot be 
utilised, since the cost of purification would exceed the 
value of the product. On the other hand, the purer 
kinds, when suitably refined, yield a graphite that is 
fully adapted to all requirements. 

The refining process may be either chemical or 
mechanical, the choice of methods depending entirely 
on the character of the associated minerals. If these 
mainly consist of coarse, stony fragments, preference 
should be given to mechanical treatment ; but if they 
are of such a character that they cannot be eliminated 
in this way, chemical methods must be employed. 
Sometimes the two systems are combined, by first 
subjecting the graphite to a rough mechanical purifi- 
cation, and then completing the operation with chemical 

The mechanical treatment consists in first removing 
as many of the impurities as possible by hand-picking, 
and grinding the remainder in edge -runner mills, along 


with water. The turbid liquid, containing the powdered 
graphite and extraneous minerals in suspension, is 
led through long launders, the sides of which are 
notched at intervals to allow the water to overflow into 
large pits. The graphite settling in the first of these 
pits contains numerous particles of the heavy associ- 
ated minerals; but that remaining suspended in the 
water and carried on to the further pits constitutes 
the bulk. The water is left to clarify completely in 
the pits, and is then drawn off, the pasty residue being 
shaped into prisms, which are compressed under heavy 
pressure, to increase their density, when partially d^. 

Although levigation will remove most of the accom- 
panying extraneous minerals, it cannot eliminate the 
ash constituents of the graphite. Experiments made 
in this direction have demonstrated that the ash 
content of the levigated graphite is exactly the same 
as that of the crude material. Whilst these ash con- 
stituents do not affect the quality of graphite for cer- 
tain of its uses, they nevertheless impair its beautiful 
black colour to a considerable degree. The chemical 
treatment necessary to eliminate these constituents is 
attended with many difficulties, the chief of which 
resides in the fact that the ferric oxide present is in a 
form that is not readily accessible to the action of 
chemicals. For this reason, attempts to purify graphite 
with crude hydrochloric acid are hardly likely to prove 
successful, since both the ferric oxide and the accom- 
panying silicates obstinately resist the action of this 

In order to obtain graphite of a high state of purity, 
the attempt must be made to bring this ferric oxide 
and the silicates into a soluble condition. This can 
be accomplished in various ways, and the choice of 


the method will depend on the purpose for which the 
graphite is intended. For example, the operations may 
either be confined to purification, or else include the 
attainment of a maximum condition of subdivision. 
When foliaceous graphite has to be treated and this 
kind of graphite cannot, in its original condition, be 
used for making lead pencils it is preferable to employ 
a method which will produce both the above results. 
The purification may consist in crushing the graphite 
to powder, and fusing this with a mixture of sulphur 
and carbonate of soda, whereby the silicates present 
are converted into soluble compounds, and the ferric 
oxide into ferric sulphide. On extracting the melt 
with water, a portion of the contained salts pass 
into solution and is carried off. The residue is then 
treated with dilute hydrochloric acid, which dissolves 
out the ferric sulphide, with liberation of sulphuretted 
hydrogen, and leaves the graphite in a very pure 
condition after w r ashing. 

In order to render foliaceous graphite suitable for 
lead pencils, a different method is pursued, but should 
only be employed in special circumstances, on account 
of the expense entailed. 

According to the process recommended by Brodie, 
the graphite, ground to coarse powder, is mixed with 
about one-fourteenth of its own weight of chlorate of 
potash, this mixture being heated, with two parts by 
weight of sulphuric to each part of graphite, in a water 
bath so long as fumes of hypo chlorous acid continue 
to be disengaged. The heating must be performed in 
stoneware or porcelain vessels, those made of any 
other materials being strongly corroded by the chlorine 
compounds formed. 

When the evolution of fumes ceases, the mass is 


allowed to cool, and is carefully washed with a large 
volume of water, the residue being then dried and heated 
to redness. During this calcination the graphite under- 
goes a peculiar change, increasing considerably in bulk 
and forming an exceedingly soft powder which, after 
another washing, consists almost entirely of chemically 
pure carbon. 

Graphite purified in this way can be used for any 
purpose for which this material is employed, and may 
be made into the finest lead pencils. However, as 
already mentioned, this process is usually too expensive 
for general application. 

The use of graphite for writing is more ancient than 
is usually supposed, having been tentatively employed 
between 1540 and 1560. It was during this period 
that the graphite mines in Cumberland were discovered ; 
and the extremely pure graphite found there soon began 
to be used as a writing material. 

Up to the close of the eighteenth century, lead pencils 
were made by selecting pure lumps of graphite and 
sawing them into thin rods, which were then encased 
in wooden sticks. Apart from their high price, these 
pencils exhibited various defects, one of the chief being 
that a stick of such pencil was seldom of uniforrh hard- 
ness throughout its length, most of them being so soft 
in parts as to make a deep black, smeary mark, whilst 
other parts would hardly give any mark at all. 

The defects inherent in native graphite are com- 
pletely removed by the method now generally employed 
in making lead pencils; and on this account the old 
process of sawing the lumps has been abandoned. 

Graphite with a fine earthy texture alone is suitable 
for lead pencils, scaly varieties being useless for this 
purpose, unless specially prepared, since they will not 


give a solid black streak. By means of the Brodie 
process, however, even the most highly crystalline 
kinds can be rendered suitable for this purpose. 
Siberian graphite is distinguished by extremely high 
covering power, and is specially preferred for the 
manufacture of pencils. Excellent varieties for this 
purpose are also found in many parts of Europe ; and 
indeed, a large proportion of all the lead pencils used 
throughout the world are made from Bohemian, Styrian 
and Bavarian graphite. 

At present, all pencils are made from ground graphite, 
the extremely finely ground and levigated material 
being kneaded into a paste with clay. This operation 
fulfils a twofold purpose, the plasticity of the clay 
increasing the cohesion of the individual particles of 
graphite, whilst the amount of clay used determines 
the hardness of the pencil. 

The larger the proportion of clay, the harder the 
pencil when baked, and therefore the paler the mark 
the pencil will make on paper. In the pencil factories, 
the clay is incorporated in special machines; and the 
operation requires extreme care, since only a perfectly 
uniform mixture will give a composition of regular 
character in all cases. 

The intimately mixed material is formed into thin 
rods, which are dried and then baked, the heat driving 
out the water in the clay and transforming it into a 
solid mass. 

An addition to this main application of graphite, 
the mineral is also used for making crucibles, chiefly 
for melting the noble metals. Crucibles of this kind 
are largely manufactured near Passau, Bavaria, and 
similar crucibles are made in England from Ceylon 


Another important use for graphite is as a coating 
for iron articles to protect them from rust. For this 
purpose, however, only the inferior kinds are employed ; 
and these can also be made up into excellent cements 
capable, in particular, of offering considerable resistance 
to the action of heat and chemicals. 

To complete the tale of the applications of graphite, 
its employment as a lubricating agent for machinery, 
especially for reducing friction in machines made of 
wood, may be mentioned. Latterly also, the finest 
levigated graphite has come into use, in admixture 
with solid fats or mineral oils, for lubricating large 
engines, for which purpose it is excellently adapted. 


Black chalk, slate black, Spanish chalk, crayon, etc., 
is not a chalk at all, in the mineralogical sense, but 
consists of clay shale of varying colour. Some kinds 
of this shale are pure black, almost velvet black, and 
these are considered the best. Others have a more 
greyish or bluish tinge and are of low value as 

The purer the black, the finer the grain of the 
material, and therefore the greater its value to the 
colour-maker. The variety obtained from Spain is 
generally admitted to be the best, and for this reason 
the name of Spanish chalk has been applied to all 
similar minerals. 

In all cases the black colour of Spanish chalk is due 
to carbon; but the particular modification of carbon 
present has not yet been accurately identified. Accord- 
ing to some, it is chiefly graphite, whereas others ascribe 
the colour to amorphous carbon. Apparently, the 


material found in different deposits contains either one 
or the other of these modifications of carbon. 

Deposits of black chalk are fairly plentiful, but in 
many of them the material is so contaminated with 
extraneous minerals that a somewhat troublesome 
method of preparation is needed to fit them for the 
purpose of the draughtsman. With this object, the 
native product must be ground extremely fine, and 
the powder levigated; and owing to the expense of 
these processes, they are now seldom used, it being 
possible to obtain a good black chalk far more cheaply 
than by levigating the natural material. 

This artificial black chalk is prepared by mixing 
ordinary white chalk, or white clay, with a black 
colouring matter, shaping the mass into prisms, and 
sawing these into suitable pieces when dry. The 
white pigment may either be mixed with some very 
deep black substance, such as lampblack, or stained 
with an organic dyestuff , which is, in reality, not black, 
but either very dark blue or green. 

The usual colouring matter used with white chalk 
is lampblack, mixed to a uniform paste with thin glue, 
a suitable amount of clay or chalk being incorporated 
with the mass. The production of a perfectly homo- 
geneous mixture entails subjecting the paste to a 
somewhat protracted mechanical treatment. When 
the mass has become perfectly uniform throughout, 
it is shaped into prisms, which are exposed to the air 
to dry and are then cut up with a saw. Instead of 
prisms, the mass can be shaped into thin sticks, which 
dry more quickly. 

A very handsome black chalk can be made, with 
comparatively little trouble, by treating chalk with a 
suitable quantity of logwood decoction previously 


mixed with sufficient green vitriol solution to render 
the liquid a deep black. This liquid is added to the 
dry chalk, intimately mixed therewith, and the pasty 
mass shaped into sticks. The colouring agent may be 
replaced by a solution of logwood extract blackened 
by the addition of a small quantity of chromate of 
potash ; or black dyestuffs may be used. 



MENTION has already been made of the great con- 
fusion prevailing in the nomenclature of pigments, 
and that many of these are put on the market under 
a variety of names taken from different languages. 

Although the number of the earth colours is far 
smaller than that of the artificial colouring matters, 
the nomenclature is in a no less confused condition. 

Most frequently, earth colours are named after the 
localities where they are either discovered or prepared, 
in combination with the word indicating the colour 
of the product for example : Cologne white, Vienna 
white or the term " earth " (Verona earth, Veronese 
green, etc.). Whilst these names give, to some extent, 
an indication of the nature of the pigment, others 
have no reference to it at all; such as colcothar, bole, 
umber, etc. Finally, a number of other names in 
use are calculated to produce the impression that the 
earth colours in question are of an entirely different 
nature to their real one. As an example, we may 
cite the name " French chalk," which is not a chalk 
at all, but consists of the mineral talc. Black chalk, 
again, is not chalk (calcium carbonate), but a black 
shale; and graphite is often termed blacklead, 
although it contains no lead at all, and the name is 



merely a survival from the time when pencils of 
metallic lead were used for drawing. 

In order to bring some kind of order into the various 
names which are applied to the earth colours, a list 
of those in current use is appended. Many of these 
names, it may be stated, have been selected in a purely 
arbitrary manner, some manufacturers, for instance, 
selling ordinary chalk under a variety of foreign 
names, for the purpose of thereby obtaining higher 
prices. These borrowed names would seem to be 
superfluous, to say the least. Pure and properly 
levigated chalk is the same article everywhere, whether 
prepared from English, French or German limestone; 
and in all cases the simple name, " chalk," with an 
explanatory " single," " double," or " triple " levi- 
gated, should be quite sufficient. 

In the case of earth colours that are really obtained 
of special quality in certain localities, such as terra di 
Siena, green earth from Verona, or the like, the corre- 
sponding name might be retained, even if the pigment 
did not originate from the locality in question, as a 
generic term for a pigment possessing certain properties 
and of a certain composition. 

In the following classification, the names of the earth 
colours are given in accordance with their colour and 
chemical composition. 


Carbonate of Lime : 

Chalk; levigated chalk; Vienna white; Spanish 
white ; marble white ; artists' white ; Bougival white ; 
Champagne chalk; Paris chalk; Cologne chalk; 
Mountain chalk; craie; blanc mineral; Blanc de 


Champagne; Blanc de Meudon; Blanc de Bougival; 
Blanc de Troyes; Blanc d' Orleans; Blanc de Rouen; 
Blanc de Briancon. 

Basic Carbonate of Lime : 

Vienna white ; Vienna lime ; pearl white ; whiting ; 
Blanc de chaux ; Blanc de Vienne. 

Note. The calcareous marls, consisting of carbonate 
of lime and clay, are also frequently sold under the 
above names, the same being the case with gypsum. 

Silicate of Alumina : 

White earth; pipeclay; Dutch white; Cologne 
earth ; terre d' Argile ; Argile blanc ; Terre blanche. 

Silicate of Magnesia (mineralogically, talc and 
soapstone) : 

Talc; Venetian earth; French chalk; Venetian 
white; glossy white; feather white; shale white; 
face-powder white ; Blanc de Venise, Blanc d'Espagne ; 
Blanc de fard. 

Note. Fine grades of white lead are also sold as 
Venetian white, Spanish white and shale white; but 
can easily be recognised by their weight. The term 
"prepared" white, frequently applied to earth 
colours in the trade, usually indicates that the material 
in question has been either levigated, ground or burnt 
in short, put through some kind of preparatory treat- 
ment and is therefore in frequent use for all the 

Barium sulphate : 

Heavy spar; barytes; heavy earth; mineral white. 

Precipitated colours : 
Permanent white ; blanc fixe. 



Ferric hydroxide, with admixtures of ferric oxide, 
clay, lime, ferric silicate, basic ferric sulphate, etc. 

Ochre; iron ochre; golden ochre; satin ochre 
(satinober) ; pit ochre ; vitriol ochre ; Mars yellow ; 
Chinese yellow; Imperial yellow; permanent yellow; 
terra di Siena ; umber ; Italian earth ; Roman earth ; 
bronze ochre; oxide yellow, etc. 

Yellow ochre; JaunedeMars; Terre d' Italic. 

Ferric Silicate : 

Yellow earth; Argile jaune; yellow wash. 


Ferric oxide (with alumina and silica). 
Bolus ; bole ; Terra sigillata ; Lemnos earth ; red 
chalk; raddle; Striegau earth. 

Ferric oxide : 

Colcothar; English red; angel red; Pompeii red; 
Persian red; Indian red; Berlin red; Naples red; 
Nuremberg red; crocus; chemical red; Crocus 
Mart is iron saffron ; caput mortuum ; raddle ; rouge 
de fer; Rouge de Perse; Rouge des Indes; Rouge 
de Mars; Rouge d'Angleterre. 


Ferric oxide : 

Ferric hydroxide; Ferric silicate (conf. Yellow 
Earth Colours, which are often sold under the same 


names as the browns. The paler kinds are usually 
called " pale " or " golden," such as pale ochre, golden 
ochre, etc.). Terra di Siena; burnt Siena; satinober; 
mahogany brown ; Vandyke brown. 

Ferric silicate, Clay : 

Umber; umber brown; Roman earth; Roman 
umber; Turkish brown; Sicilian brown; Cyprus 
earth ; chestnut brown ; burnt umber ; ombre ; Terre 
d'ombre ; Ombre brulee. 

Organic decomposition products : 

Cologne umber; Cologne earth; Cassel brown; 
Spanish brown; mahogany brown; Vandyke brown; 
brown carmine; Terre brun de Cologne; Brun de 
Cologne ; Brun d'Espagne ; Ombre de Cologne ; Brun 
de Cassel; Terre d'Ombre; Cologne brown. 

Asphaltum (mineral rosin) : 

Asphaltum brown ; bistre ; earth brown ; bitumen ; 
pitch brown ; Asphalte ; Brun de bitume ; Bitume. 


Ferrous oxide with silica, alumina, lime, etc. : 
Green earth ; Verona green ; Celadon green ; Verona 
earth ; Italian green ; stone green ; Bohemian earth ; 
Cyprus earth; Tyrol green; permanent green; green 
ochre; Terre verte; Terre de Verone ; Vert d'ltalie. 

Cupric carbonate : 

Malachite green; mountain green; Hungarian 
green ; copper green ; mineral green ; Tyrolese green ; 
shale green; Vert de montagne; Vert d'Hongrie. 



Cupric carbonate : 

Malachite blue; mountain blue; lazulite blue; 
azure blue; mineral blue; copper blue; Hamburg 
blue; English blue; Cendres bleues; Bleu d'azure; 
Bleu de cuivre ; Vert-de-gris bleu ; blue verditer. 


Grey clay shale : 

Mineral grey ; silver grey ; stone grey ; slate grey. 

Carbon : 
Graphite ; blacklead ; plumbago ; iron black. 

Clay shale : 

Black chalk; slate black; Spanish black; Spanish 
chalk; oil black; Schiste noir; Noir d'Espagne. 


Alabaster. See Gypsum. 
Alumina, silicate of, 21, 22 
Aluminium-potassium silicate, 

Alum sludge, 32 

, artificial ochre from, 148 

, ferric oxide pigments from, 

Ammonium salts, artificial ochre 

from, 145, 146 
Anhydrite, 19 
Anthracolite, 13 
Arragonitc, 13 
Asphaltum, 37, 38. See. also 


, brown, 174, 175 
Augite, 179 
Azurite, 33 

Ball Mills, 55-56 
Barium carbonate, 20 

sulphate. See Barytes. 
Barytes, 19, 20, 119-122 

, artificial, 133 
, correcting colour of, 121 
, detecting, in white lead, 

, low covering power of, 121 
Black chalk, 38 
earth colours, 185-196 

, trade names of, 202 

earths, 6, 38-39 

schist, 38 
Bitumen, 174, 175 
Blanc fixe, 19 

Blue earth colours, 183-184 
, trade names of, 202 

earths, 4, 33-36 
Bole, 31, 32, 152-154 
Bone breccia, 13 

Brown coal, pigments from, 175 
earth colours, 168-175 

, trade names of, 200 
earths, 5, 3638 

Calcareous marl, no, in 

tuff, 12 
Calcining, 81 

Ferric oxide, 161-164 
furnaces. See Furnaces. 

lime, 88-90 

ochre, 132-136 
Calcite, n, 12, 14, 15 
Calcium carbonate, 12, 14, 15, 16 

, action of acids on, 15 

hydroxide, 16 

sulphate. See Gypsum. 
Calc sinter, 12 

spar. See Calcite. 
Caledonian brown, 36 
Cappagh brown, 36 

Caput mortuum. See Colcothar. 
Carbon brown, 37 

in limestone, 16 

Cassel brown, 37, 38, 174 
Celadon green. See Green earth. 
Chalk, 13 
, black, 194-196 

, colour of, 103 
, correcting colour of, 104, 

105, 106 

, covering power of, 106 

, grinding, 101 

, impurities in, 103, 104 

, precipitated, 107-103 

, preparation and properties 

of, 98-106 
Classification of earth colours, 

Clay, 21-23 




Clay, formation of, 113 
, impurities in, 114-119 

in ochre, 128 

, levigating, 114-117 
Colcothar, 160, 161, 162 
Cologne earth, 173, 174 
Commercial nomenclature of 

earth colours, 197-202 
Crushers and Breakers, 43-45 
Crushing, 77-80 

machinery, 43-60 

Disintegrators, 58-60 
Distemper, weatherproof, 94 
Dolomite, 18 

Draining and Drying, 66-77 
Drying appliances, 73-77 
Dyestuffs for improving earth 
colours, 85 

Edge runners, 48-55 
English red, 160 

Ferric hydroxide in ochre, 128- 

oxide, artificial ochre from, 


as by-product, 30 
, burnt, 158-164 

, calcining, 161-164 

in lime, detection of, 

, native, as pigment, 

- pigments from alum 
sludge, 164-167 

, range of colours, 29 

shading, 28 
, violet shades from, 

Ferrous sulphate, artificial ochre 

from, 139-143, 146-148 
Filter- cloths, cleaning, 72 
Filter-presses, 70-73 
Furnaces, calcining, 158, 162, 

163, 166 

Granulator, 43 
Graphite, 38, 39, 185-194 

as a lubricant, 194 

as anti-corrosive, 194 

Graphite in the manufacture of 
lead pencils, 191-193 

for crucibles, 193 
, refining, 189-192 

Green earth, 176-180 

, artificial, 180 

, improving, 178 
colours, 176-184 
, trade names of, 201 

earths, 5 

Grey earth colours, trade names 

of, 202 

Grey earths, 38 
Gypsum, 18/19, in, 112 

Heavy spar. See Barytes. 
Hematite, 155 

, brown, 23, 30, 31 

, red, 28, 30 
Hydro -extractor, 66-70 

Improving earth colours, 84, 85 
Indian red, 29, 160 
Iron cream, 29 

glance, 154 

in limestone, 17 

ore, bog, 25, 31 

, micaceous, 28 
Ironstone, brown, 23, 24, 25 

, clay, 24 
, red, 28-30 

Kaolin, 21, 22, 112-119 

Lazulite, 183 

Lemnos earth. See Bole. 

Levigation, 60-65 

Lime, absorption of carbon 

dioxide by, 93 

, action of, on casein, 94 

, , on colours, 93, 98 

, calcining, 88-90 

. , caustic, preparation of, 

, double compound of oxide 

and carbonate, 93 

from mussel shells, 98 
, impurities in, 91, 92 

in clay, 22 

, eliminating, 117- 

in ochre, 129 



Lime in the preparation of arti- 
ficial ochre, 140-144 
, moulding, 96-98 

, quick, 16 

, slaked, 16 

Limestone, 11-18 

, suitability of, for colour- 
making, 92 

Limonite, 25 

Magnesia, carbonate of, 123, 124 

in lime, 91 

in limestone, 17 
Magnesium silicate, 21 
Malachite, 35 

-blue, 183 
Marble, n, 14, 15 
Minerals, testing for suitability 

as pigments, 172 
Mine sludge, 32 
Mixing earth colours, 81-84 
Moulding, 85, 86 
Mountain chalk, 12 

milk, 12 

Muffle, burning ochre in the, 

Muriacite, 19 
Muschelkalk, 13 

Ochre, 24, 25, 26 

, blue. See Vivianite. 

, calcining, 132-136 

, English, 138 
, green, 180 

, pit, 148-150 

, Roman, 137, 138 

, Siena, 137, 138 

.testing, 130-132 

, toning with chalk, 144 

, toning with clay, 144 

, vitriol, 146-148 
Ochres, 128-150 

, artificial, 138-146 

as by-products, 146-150 
, burnt, 158-164 

from various deposits, 136- 


, Italian, 137, 138 

Oolithic limestone, 13 
Organic matter in lime, 91 

Pastel crayons, 126 

Pearl white, 94 
Permanent white, 19, 122 
Pipeclay. See Kaolin. 
Preparation of colour earths, 40- 

Pulverisers, 56-58 

Raddle, 29, 155-158 
, impurities in, 156 

.testing, 157 

Raw materials for earth colours, 

Red earth colours, 151-167 

, trade names of, 200 

Red earths, 4, 27-33 

Sampling raw earths, 9 

Selenite, 18 

Shading pigments with perma- 
nent white, 19 

Siena, Terra di, 25, 26, 27, 168- 

, . See also Italian 


Siderosilicate, 171 

Sifting, 77-80 

Soapstone, 20, 21. See also 

Spanish brown, 174 

Sprudelstein, 15 

Steatite, 20, 21, 125, 126 

Stamps, 45-48 

Talc, 20,21, 124, 125 
Terra sigillata. See Bole. 
Testing purity of raw earths, 

Trade names of earth colours, 


Ultramarine, 33 
Umber, 36, 170-174 

, Cologne, 173, 174 

, true, 170-173 

Vandyke Brown, 38, 174 
Vermilion, 151 

Verona earth. See Green earth. 
Vienna white, 95-98 
Vivianite, 33, 34, 184 

White earth colours, 87-126 

206 INDEX 

White earth, trade names of, Working earth colour deposits, 9 

in, 198, 199 

White earths, 4 Yellow earth colours, 127-150 

White raw materials and pig- , trade names of, 100 

mentary earths, 11-23 Yellow earth, 150 

Witherite, 20 Yellow earths, 4, 23-27 


The Manufacture of Paint 




Second Edition Revised and Enlarged 












XVI. Cost Charges Cost of Handling Carriage and 
Delivery of Goods Cost of Materials Machinery 
as affecting Manufacturing Cost Electricity as 
Motive Power Manufacturing Oncost Prices 
The Future of the Industry. 

Price 12s. 6d. net (Post Free, 13s. 3d. Home and Abroad). 


8 Broadway, Ludgate, London, E.G. 4 

The Chemistry of Pigments 




Demy 8vo. 5 Illustrations. 280 Pages 

Chapter I. Introductory 

Light White Light The SpectrumThe Invisible Spectrum 
Normal Spectrum Simple Nature of Pure Spectral Colour 
The Recomposition of White Light Primary and Comple- 
mentary Colours Coloured Bodies Absorption Spectra. 

Chapter II. The Application of Pigments 

Uses of Pigments : Artistic, Decorative, Protective 
Methods of Application of Pigments : Pastels and Crayons, 
Water Colour, Tempera Painting, Fresco, Encaustic Painting, 
Oil-Colour Painting, Ceramic Art, Enamel, Stained and 
Painted Glass, Mosaic. 

Chapter III. Inorganic Pigments 

White Lead Zinc White Enamel White Whitening Red 
Lead Litharge Vermilion Royal Scarlet The Chromium 
Greens Chromates of Lead, Zinc, Silver and Mercury 
Brunswick Green The Ochres Indian Red Venetian Red 
Siennas and Umbers Light Red Cappagh Brown Red 
Oxides Mars Colours Terre Verte Prussian Brown 
Cobalt Colours Coaruleum Smalt Copper Pigments 
Malachite Bremen Green Scheele's Green Emerald Green 
Verdigris Brunswick Green Non-arsenical Greens 
Copper Blues Ultramarine Carbon Pigments Ivory Black 
Lamp Black Bistre- Naples Yellow Arsenic Sulphides : 
Orpiment, Realgar Cadmium Yellow Vandyck Brown. 

Chapter IV. Organic Pigments 

Prussian Blue Natural Lakes Cochineal Carmine Crim- 
son Lac Dye Scarlet Madder Alizarin Campeachy 
Quercitron Rhamnus Brazil Wood Alkanet Santal Wood 
Archil Coal-tar Lakes Red Lakes Alizarin Compounds 
Orange and Yellow Lakes Green and Blue Lakes Indigo- 
Dragon's Blood Gamboge Sepia Indian Yellow, Puree 
Bitumen, Asphaltum, Mummy Index. 

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